Better exception messages

Moving the message from toString to exception message instead, since that is what is used automatically in many places.

This will make the exception look something like:
Order.set: 
Data: [{"message":"Ogiltigt alternativ f\u00f6r\u00a0\/language: \u201d\u201dfoo\u201d","uri":"urn:#\/language"}]
Request: {"jsonrpc":"2.0","method":"Order.set","id":"d7b6e0b0-2ead-4a47-899c-812f4b517f67","params":[null,{"language":"foo"}]}
No, that can't be right anymore.  Must use $this->request->errorMessage.