Emit error and error_code

[mstoykov]

Connected issue for the original k6 api:

Currently, if an error (like timeout) happens during the creation of the websocket, k6 will not emit status=0 or tags error and error_code as the HTTP module, but will just no emit the status tag, call the error handle if defined and end continue executing javascript.

This will require for the error_code code to be made exported and possibly moved to a different module, which will impact grafana/k6#2008, so we might want to merge that first.