I would like to suggest adding support for error codes in command results and (probably) exceptions, so handling specific type of errors is easier.
Comparing error messages to a predefined string seems weak to me. While current driver has only one such spot (the 'commandResult.ErrorMessage == "not master"' check in MongoDatabase.cs, I don't know if this is the only and proper message to close connection for), not having access to error code makes writing robust client-side code harder.