ignore emacs lock files and exception handling for watchers

emacs creates 'lock files' (``'.#*'``) for modified files ([ref](http://www.gnu.org/software/emacs/manual/html_node/elisp/File-Locks.html)). They are invalid links with the purpose of preventing simultaneous editing of the same file. But since they are invalid, `os.stat` can't access them.

This PR adds that pattern as default for ``IGNORE_FILES``.

Also exception handling for ``*_watcher`` generators are added so that generators won't quit unexpectedly.