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]>

The util::ResultPtr class is a wrapper for util::Result providing syntax sugar
to make it less awkward to use with returned pointers. Instead of needing to be
deferenced twice with **result or (*result)->member, it only needs to be
dereferenced once with *result or result->member. Also when it is used in a
boolean context, instead of evaluating to true when there is no error and the
pointer is null, it evaluates to false so it is straightforward to determine
whether the result points to something.

This PR is only partial a solution to bitcoin#26004 because while it makes it easier
to check for null pointer values, it does not make it impossible to return a
null pointer value inside a successful result. It would be possible to enforce
that successful results always contain non-null pointers by using a not_null
type (bitcoin#24423) in combination with
ResultPtr, though.