Added support for custom error text

Hello!

I have made some changes to provide real error text returning, but not only "Internal Error".
For examlpe, code

raise HTTPError(403)

will return this json:

{"error": {"message": "HTTPError: HTTP 403: Forbidden", "code": -32000}, "jsonrpc": "2.0", "id": "r0ql48xq"}

PS. Sorry for my english :)