Handle cases when file watch is not supported (Bug #13881)

When a remote folder is accessed, this error is printed:
(thunar:9378): thunar-CRITICAL **: thunar_file_watch: assertion '...

Upon leaving that folder, Thunar crashes:
ERROR:thunar-file.c:3929:thunar_file_unwatch: code should not be reached

I found out that a call to g_file_monitor in thunar_file_watch has no
error handling, and in the case of remote folders this error happens:
Failed to create file monitor: Operation not supported by backend

So when ThunarFolder is finalized and calls thunar_file_unwatch, its
file_watch will be NULL and _thunar_assert_not_reached kills Thunar.