Returning errors from plugins

[Richard Hughes <hughsient gmail com>]

Hi all,

I've just merged a commit to master which emits a critical warning if
plugins return GErrors with a domain other than GsPluginError. The
correct domain and code is required so we can properly add the
relevant in-app notifications. Please try to use the correct
GS_PLUGIN_ERROR_ enum when creating errors and then the notification
text will be correct. I've added a few helpers to gs-utils.c which
means the error can be fixed up very easily in most cases.

I've not pushed it to 3-20 as it's likely quite a few plugins will
need fixing up, although I think I've fixed most of the in-tree
plugins.

Richard