General/XMLRPC: Useful error messages

When our debugging tools fails at `authenticated_xmlrpc_request_for_jetpack_passes_connection_test` that often suggests the failure happened when verifying the signature. When the issue isn't obvious, it would be helpful if xmlrpc would respond with something more notable than `[faultCode] => -32601
[faultString] => server error. requested method jetpack.testConnection does not exist.`

Start at authenticated_xmlrpc_request_for_jetpack_passes_connection_test and trace the XML-RPC response when jetpack.testConnection is reported as missing. Define what additional signature-verification context the error should expose, then verify the resulting response against the debugging failure described in the issue.

Written by the indexing model from the issue text.