handle too-many-open-files err in fswatcher.New

calling fswatcher.New when the OS has too many files currently open currently results in `fswatcher: error creating watcher: too many open files`, which can cause the top level application to crash.

maybe a better behavior here is to fallback to polling and trying to watch again later?

open to suggestions!

The entry point named is fswatcher.New; begin by tracing how it handles the OS “too many open files” error and how its caller reacts. Define the intended fallback or retry behavior and a reproducible test for it; done requires agreement on that behavior and verification that the top-level application no longer crashes.

Written by the indexing model from the issue text.