Include "source" field in error responses from mongos

When a mongos returns an error from a remote node there is no indication of where the error originated. This is requested by drivers, so that they can determine if the mongos is malfunctioning versus a mongod instance. This would also be helpful for debugging and administration purposes. Right now, when errors such as InterruptedAtShutdown are sent from a mongod node through mongos drivers will follow the sdam spec and disconnect from mongos when this isn't necessary.

This ticket suggests creating a "source" field (or similarly named) for error responses that encodes this information.