goa_warning is meant for goa-daemon, and it does not really add any
value in this case. Switching to g_warning lets us cut down on the
number of symbols that are exported from libgoabackend.
https://bugzilla.gnome.org/show_bug.cgi?id=693261
With hard coded colors, when an account is selected in the tree view,
the text shows up as black on blue, instead of white on blue.
Fixes: https://bugzilla.gnome.org/671980