Only check for IsWatchedFile if a file was found.

There are some situations where the e.FullPath filename isn't found by
the virtual file system provider (it doesn't exist, etc.) so we need to
account for that before calling views.IsWatchedFile.
Merge pull request #709 from aaronlerch/razor-fsw-fix

Only check for IsWatchedFile if a file was found.