BF: Handle case when inotify watch is auto deleted on file deletion

When a file is deleted, the watcher for it is automatically removed.
FilterPyinotify is detecting a new file being created with the same
name, and in turn attempts to remove the watch for the deleted file
(which has already been removed automatically).

Also, IN_IGNORED events are generated when a file is deleted, but these
weren't being caught, causing an non-existent file path to be passed to
FilterPyinotify._process_file (which caught the exceptions in
Filter.getFailures).

Closes #660