When authentication fails I get the following exception message currently:

> Mongo::Auth::Unauthorized (User foo (mechanism: aws) is not authorized to access test (auth source: $external, used mechanism: MONGODB-AWS, used server: localhost:14421 (PRIMARY)): Received authentication for mechanism MONGODB-AWS which is unknown or not enabled (2: BadValue) (on localhost:14421))

Here, I expect that (2: BadValue) is part of the server-generated message since the driver doesn't have a mapping from codes to code names.

Here is an exception message produced for OperationFailure for comparison:

> Mongo::Error::OperationFailure (no such command: 'pingx' (59) (on localhost:14421))

In this message, 59 is the code returned by the server but the formatting of 59 in the parentheses is supplied by the driver, and the server-supplied text is just "no such command: 'pingx'".

We should clearly indicate in exception messages produced by the driver which part of the message came from the server and which part was composed by the driver.