fix(server): Log a backtrace when RPC fails. Closes #1223

[rrw-zilliqa]

Log an exception when an RPC error occurs, to cue observers in on where the issue might lie. It would be better in future to decode these properly and return a meaningful error message in a well-formed JSON reply, but for now this is at least better than claiming that all errors are RPC format problems. I have avoided returning valid JSON in case it causes us to leak security-sensitive data to callers.