filesystem: Better error message when browsing fails
filesystem: Better error message when browsing fails

We try to keep the original error domain and code, so that
interested front-ends can act upon the errors. For example,
a G_IO_ERROR/G_IO_ERROR_NOT_MOUNTED error could trigger something
to mount the item.

https://bugzilla.gnome.org/show_bug.cgi?id=725203

grilofw/grilo-plugins,MikePetullo/grilo-plugins,jasuarez/grilo-plugins,MathieuDuponchelle/grilo-plugins,GNOME/grilo-plugins,GNOME/grilo-plugins,vrutkovs/grilo-plugins,MathieuDuponchelle/grilo-plugins,vrutkovs/grilo-plugins,MathieuDuponchelle/grilo-plugins,MikePetullo/grilo-plugins,vrutkovs/grilo-plugins,grilofw/grilo-plugins,MikePetullo/grilo-plugins,vrutkovs/grilo-plugins,jasuarez/grilo-plugins,MathieuDuponchelle/grilo-plugins