Don't error on watching non-existent files

calling .realpath will throw a Errno::ENOENT, but as per pr #147,
we would rather not watch non-existent files.

Change-Id: Ic65fd9c08692c0d69c789a21c16d288e3f7acab7
Merge pull request #162 from cbrunsdon/who_watches_the_watchers

Don't error on watching non-existent files