Cleanup code with correct usage of $err and errmsg

[apkar]

Mongo has $err and errmsg in response from the server. Usually, $err is for the error code and errmsg is for error description. Our error handling is not consistent. At least it is less consistent than Mongo.

[senthil-db-expert]

@apkar please assign this to me.
Will cleanup and make them uniform using seterror function