NPE on poorly defined plugin namespace

[dirtyvagabond]

Trying to use the protocol from an incorrectly namespaced plugin gives an ugly NPE stack trace.

E.g., a plugin where the protocol def is held in (ns X-Y.protocol.rows) rather than (ns Y.protocol.rows), and a step like:

out.csv <- in.csv [Y]

Presumably this could be reproduced by simply not having the protocol def at all.

Instead of the NPE we should provide a helpful error and graceful exit