validateHandlers not verbose enough

[KeKs0r]

I was running into an issue where I misconfigured my handlers.

This is usually caught in: https://github.com/martyjs/marty-lib/blob/master/src/store/validateHandlers.js#L15

But since it is within a Flux.Dispatcher.dispatch, the error is caught and swallowed. I would therefore prefer it logging the error instead of throwing it.

I could also do the PR if this is agreed upon

[jhollingworth]

Yeah, makes sense. PR away 👍