Mapper: Correct Unknown Method Error Handling

Since refactoring in commit, 3c278975c59cf8dbcfdfdf1184e4cf9c89de7ce3
we have experienced some issues when using e.g. d-feet with banshee, as
described in BGO#725446.

This patch corrects a small mistake in the refactoring process, in the
creation of an org.freedesktop.DBus.Error.UnknownMethod message.