MTP: Improve error messages.
MTP: Improve error messages.

halfline/gvfs,philipl/gvfs,gicmo/gvfs,xkahn/gvfs-cmis,xkahn/gvfs-cmis,gicmo/gvfs,xkahn/gvfs-cmis,philipl/gvfs,gicmo/gvfs,philipl/gvfs,gicmo/gvfs,halfline/gvfs,xkahn/gvfs-cmis,halfline/gvfs