Key error when a message from unknown plugin arrives

When the collect master receives a message from unknown plugin, it raises a key error and terminates the connection. This, while not generally happening, should be handled more gracefully.

How to reproduce: enable the plugin in the client, in the database (in known plugins ‒ in the debug setup all are enabled by default) and do not enable it in the master's configuration. Choose a plugin that does send a message during startup to get the error right away (eg. flow).