g_file_set_contents(): don't allocate display name

g_file_set_contents(): don't allocate display name

g_file_set_contents() sets a GError in the event of various failures
that count occur.  It uses g_filename_display_name() in order to get the
filename to include in the messages.

Factor out the error handling to make it easier to allocate the display
name only when we need it (instead of allocating it every time).

https://bugzilla.gnome.org/show_bug.cgi?id=701560