Problem: libzmq doesn't implement ERROR commands

[hintjens]

Solution: the stream engine should send back an ERROR command (see RFC 23) when there's a protocol or access error. The ERROR command carries a text explanation, which can be "protocol error" or "access refused".

[sigiesec]

In case of ZAP errors, this is done now, but not in other cases, e.g. in case of a mechanism mismatch.