Add util::Result support for returning more error information. For better error
handling, adds the ability to return a value on failure, not just a value on
success. This is a key missing feature that made the result class not useful
for functions like LoadChainstate() which produce different errors that need to
be handled differently.

This change also makes some more minor improvements:

- Smaller type size. util::Result<int> is 16 bytes, and util::Result<void> is 8
  bytes. Previously util::Result<int> and util::Result<void> were 72 bytes.

- More documentation and tests.

Previous commit causes the warning to be triggered, but the suboptimal return
from const statement was added earlier in 517e204.

Warning was causing CI failure https://cirrus-ci.com/task/6159914970644480

Add util::Result Update method to make it possible to change the value of an
existing Result object. This will be useful for functions that can return
multiple error and warning messages, and want to be able to change the result
value while keeping existing messages.

…ainstate

Add util::Result support for returning warning messages and multiple errors,
not just a single error string. This provides a way for functions to report
errors and warnings in a standard way, and simplifies interfaces.

The functionality is unit tested here, and put to use in followup PR
bitcoin#25722

Suggested by Martin Leitner-Ankerl <[email protected]>
bitcoin#25722 (comment)

Co-authored-by: Martin Leitner-Ankerl <[email protected]>

Add new InfoType field to util::Result which unlike SuccessType and FailureType
is useful for returning extra information in a result regardless of whether the
function succeeded for failed.

This will be used to return FlushStatus values from libbitcoinkernel functions
indicating if flushed data in addition to whether they succeeded or failed.

The InfoType field is stored directly in the util::Result object, unlike
FailureType and MessagesType which are stored in dynamically allocated memory
and require an extra memory allocation if they are accessed.

Return FlushResult instead of bool from BlockStorage FlushUndoFile,
FlushBlockFile, FlushChainstateBlockFile methods and update all callers of
these methods to use the FlushResult type internally and provide context
information for the flush failure. Three callers:
BlockManager::FindNextBlockPos, BlockManager::WriteUndoDataForBlock, and
Chainstate::FlushStateToDisk will be updated in upcoming commits to bubble
results up to their callers.

Return fatal errors from BlockManager methods that write block data. Also
update callers to use the new result types. The callers will be changed to
bubble up the results to their callers in subsequent commits.

Use result.Update in CompleteChainstateInit() and
ChainstateManager::ActivateSnapshot() so it is possible for them to return
warning messages (about flush failures) in upcoming commits.

CompleteChainstateInit() was previously changed to use util::Result in bitcoin#25665
and ChainstateManager::ActivateSnapshot() was changed to use it in bitcoin#30267, but
previously these functions only returned Result values themselves, and did not
call other functions that return Result values. Now, some functions they are
calling will also return Result values, so refactor these functions to use
result.Update so they can merge results and return complete error and warning
messages.

Return fatal error and interrupt status from LoadBlockIndex functions and
update callers to use new result types.

Return fatal errors from the Chainstate::FlushStateToDisk method and several
small, related methods which wrap it: ForceFlushStateToDisk, PruneAndFlush,
ResizeCoinsCaches, and MaybeRebalanceCaches.

Also add nodiscard annotations so callers do not accidentally ignore the result
values. Callers in init and rpc files are updated to explicitly ignore the
flush results, and other callers (AcceptToMemoryPool, ProcessNewPackage,
DisconnectTip, ConnectTip, ActivateBestChainStep, ActivateSnapshot,
MaybeCompleteSnapshotValidation) are updated to store the results in this
commit, and will be updated in upcoming commits to bubble results up to their
callers.

Return fatal errors from AcceptToMemoryPool ProcessNewPackage,
ProcessTransaction, MaybeUpdateMempoolForReorg, and LoadMempool functions.

Also add nodiscard annotations so callers handle the result values. Two callers
ActivateBestChainStep and InvalidateBlock will be updated in upcoming commits
to bubble results up to their callers.

…nctions

Return fatal errors from ActivateSnapshot, MaybeCompleteSnapshotValidation, and
ValidatedSnapshotCleanup functions. Also add nodiscard annotations so callers
handle the result values. One caller, ConnectTip, will be updated in an
upcoming commit to bubble results up to its callers.

Return fatal errors from ConnectBlock, ConnectTip, DisconnectTip, and
InvalidateBlock. Also add nodiscard annotations so callers handle the result
values. Three callers: ActivateBestChainStep TestBlockValidity, and
CVerifyDB::VerifyDB will be updated in upcoming commits to bubble results up
to their callers.

…nctions

Return fatal errors from ActivateBestChain, ActivateBestChainStep, and
PreciousBlock functions.  Also add nodiscard annotations so callers handle the
result values. Two callers, ProcessNewBlock and LoadExternalBlockFile, will be
updated in an upcoming commits to bubble results up to its callers.

Return fatal errors from AcceptBlock, ProcessNewBlock, TestBlockValidity,
LoadGenesisBlock, and LoadExternalBlockFile. Also add nodiscard annotations so
callers handle the result values.

Return fatal error ImportBlocks function and add nodiscard annotation.

Return ConnectBlock errors from VerifyDB.