Allow supplying an error catcher component

[glenjamin]

This is the first cut for error catching facilities in react-proxy.

This works on classic and class components, without breaking any existing tests.

Because the error component is optional, most of the tested code paths don't actually touch the new code. I wasn't sure how many tests to expand to cover this, so I figured I'd show what I have so far.

As this is a nice incremental change, I think it's useful to aim to merge this without pure component support initially - that might require more invasion changes.

For pure components, I think that https://github.com/gaearon/react-proxy/blob/c019762362a4936ed86d6bb89ad6a57e13de6c06/src/createClassProxy.js#L65-67 might have to change to use detection (like with

react-proxy/src/createClassProxy.js

Line 104 in c019762

if (InitialComponent.prototype && InitialComponent.prototype.isReactComponent) {

) instead of a fallback.

One thing I did wonder about just now - what happens if you attempt to replace a component defined with one approach, with a component defined using a different approach?