update bsb watcher to support -build-library

Merge branch '3.1.0' of github.com:bucklescript/bucklescript into 3.1.0

windows binary

changes

Use a new mkdirp + always call it

We used to catch errors and retry, but I feel like we weren't resetting
things correctly. This seems to work.

cc @Schmavery @jaredly

Also expose berror

Bump version for opam publish

Update version number for opam test

Sigh stuff...

New install script to avoid node dep