Don't cache exceptions in optimistic functions.
Don't cache exceptions in optimistic functions.

I originally added the exception-caching functionality in order to avoid
calling files.stat repeatedly for files known to be missing, but now that
we're using statOrNull, knowledge of missing files (as indicated by
statOrNull returning null) is being properly cached.

The reason it's dangerous to cache exceptions is that (for example) when
an ENOENT exception indicates the file is missing, there will be no more
change events for that file, effectively making the exception permanent,
even if the file comes to exist at a later time.

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