Add the "triggering mapper" to the configure_mappers error message.

Add the "triggering mapper" to the configure_mappers error message.

There are cases where the originating mapper name is not
present in the exception message, such as relationship initialization
against an unmapped class.   Ensure the originating mapper is named
in the string output.

Pull-request: https://github.com/zzzeek/sqlalchemy/pull/298
Change-Id: I9f23bfa90b26dde9229ab7ec812eec9ceae48153