Show the error overlay on page refresh

[calesce]

Continuing the discussion from gaearon/react-hot-loader#390:

It can sometimes be confusing when you don't see the overlay after refreshing, because you'll be working with the most recent valid webpack build without knowing that there's still an unresolved error until you look back at the console.

I tested with create-react-app, and their overlay appears on refresh (relevant code seems to be here), so that solution might be applicable here.

I'll be happy to do the PR for this if people agree this is a good idea.

[glenjamin]

I think this is what #132 does.

It'll be in the next release - which should be in the next day or two.

[calesce]

Should've looked harder, thanks!