Add a useful error message for IPC URI length failure

The docs have this:

> Once the minion starts, you may see an error like the following::
>> zmq.core.error.ZMQError: ipc path "/path/to/your/virtualenv/var/run/salt/minion/minion_event_7824dcbcfd7a8f6755939af70b96249f_pub.ipc" is longer than 107 characters (sizeof(sockaddr_un.sun_path)).


When I encountered this problem the only unusual output I saw was the message "Exiting on Ctrl-C" - very misleading. This patch ought to fix this.