Null message string in errored reply causes NPE

There are two parts to this issue:
1. When a responder session throws an exception with no message string, the resulting error reply also does not have any error message string. This seems to be a violation of the json-rpc spec, section 5.2. (Only data may be omitted.)
2. When a requester session receives an error reply with no message string, it throws an NPE while parsing the message string.

Need to fix both sides.

• Make responder always add some error message string.
• Make requester resilient to receiving null message strings.