golang package for enhanced errors that capture file and line number information, the call stack, relevant identifiers, and a handle back to the original error for type switching.

Replacement for fmt.Errorf(...):

err := errors.Errorf("Failed to do something; here's a value %v",len(data))
fmt.Printf("%#v\n",err) // Prints full stack trace.

Wrap another function's error:

_, err := net.Dial( "tcp", "localhost:8080" )
if err != nil {
    return errors.Go( err )
}

Obtain the original error in order to type switch:

func connect() error {
    _, err := net.Dial( "tcp", "localhost:0" )
    if err != nil {
        return errors.Go( err )
    }
    return nil
}

err := connect()
switch errors.Original( err ).(type) {
    case *net.OpError:
        // We know the original type!
}

See the godoc link for more information.