Use zmq.error.ZMQError in relay.py instead of zmq.core.error.ZMQError
Stub out the x509 backend when m2crypto isn't available

Since m2rypto and m2ext dependencies are not enforced, it is a likely
scenario that someone install fedmsg without them, which leads to crash,
cries and unhappiness. With this commit, fedmsg-logger can still run
while still complaining that it cannot check the signatures.