When a command fails, the response object currently contains various combinations of the following fields:

'err'
'errmsg'
'assertion'
'code'
'assertionCode'

For example, see the return value for:
db.runCommand(

)

This is a bit awkward from a client perspective. Can we normalize on 'err' and 'code'? If the different fields indicate different kinds of errors, we should specify that in yet another field, e.g., 'errorType'.