API break request for file monitoring

[Alexander Larsson <alexl redhat com>]

This bug:
http://bugzilla.gnome.org/show_bug.cgi?id=508564

requests and addition of a GError to g_file_monitor_directory and
g_file_monitor_file. 

Its imho, correct, but does break API which some users have started
using. I'd like to change this, but I'm not sure if that is acceptable
at this point.