JSON RPC exception suppressing

There are quite a few areas in the JSON RPC module where exceptions are being suppressed. I believe we should be raising exceptions here, or at the very least providing more information to the user that their request encountered a failure.

I'm currently testing exception raising in [this branch](https://github.com/dbcli/mssql-cli/tree/ellbosch/jsonrpc-exception-tests), and have also created a test build which runs every day to see if new exceptions are raised. As of the time of this writing, no exceptions have been raised.