Error classes are a leftover from the days of "rich" error objects.
New code should always use ERROR_CLASS_GENERIC_ERROR.  Commit
b7b9d39a..7c6a4ab8 added uses of ERROR_CLASS_DEVICE_NOT_FOUND.  Replace
them.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>