file-helpers: propagate error to caller from rb_uri_mkstemp
So, during metadata write to tracks in dirs without write access, we get the proper 'Permission denied'
error dialog, rather than the less accurate below dialog;
and a needless assertion warning.
(rhythmbox-metadata:111574): GLib-GIO-CRITICAL **: 15:18:59.553: g_output_stream_close: assertion 'G_IS_OUTPUT_STREAM (stream)' failed