Generate a more informative exception string; this is useful for debugging applications.

Generate a more informative exception string; this is useful for debugging applications.

Security note: this opens up another path for external data (in rpc_error) to flow through the application. I think this is safe, assuming that the bitcoind process, and the connection to that process, are trusted. Typically, rpc_error is completely generated by bitcoind, not containing any information from external parties.