Use ZMQError from the external api, not the internal one.
Add fedmsg.crypto to the installed packages
Listify the endpoints in tail_messages

In some places of the code, single enpoints are transformed into lists,
it seemed only fair to do this here as well.
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.