Skogul should error when no defined handlers

[sklirg]

There is no check for if a receiver has a defined handler during configuration parsing. This is also made more complex by the fact that handlers might have different key names and/or structures depending on the receiver (i.e. handlers: map[string]string for HTTP receiver and handler: string for others).

We should have a check in the configuration parsing/applying which stops skogul if it is missing a handler for a receiver.

When a receiver receives data with no handler, skogul panics because of a nil handler. So currently, this behaviour is not seen before skogul receives some data.

[KristianLyng]

I'd consider this a task for Verify() for each individual receiver, but maybe we want to modify the Receiver interface to enforce a HasHandler() check? Since Verify is, and should remain, an optional interface, and there's no generic way of testing this for the reasons you mentioned. Enforcing it through the interface would make it hard/impossible to forget to check.