make error_message behave like message

I wanted to replace many stop "error message" and call message("error", "message") stop 1 calls by call error_message("error", "message"). But that does not work and I think it should. I thus will change the error_message behaviour to be similar to message (actually calling message and passing on the arguments). It will be backward compatible and in mHM, error_message so far is not used anyways.