Implement error overlay

[SpaceK33z]

What kind of change does this PR introduce?
Feature

Did you add or update the examples/?
Yes

Summary
This adds an error overlay in the browser, like create-react-app has. It works very much like create-react-app's error overlay. In fact, I copied much code from it 😅 .

I asked on Twitter if people would like this feature, and everyone forgot Trump for a while people were happy.

The error overlay is only enabled when you add the option overlay: true in your devServer config. It could be enabled by default, but that would kinda be a breaking change so maybe in v3.

Furthermore, it will only show errors, not warnings. I'm not sure yet if we should display warnings the same way. If we would do that, there also needs to be a way to disable it.

Does this PR introduce a breaking change?
Nope

[SpaceK33z]

Anyone wants to test this? Info on how to test a PR.

[STRML]

A few comments:

1. overlay option works fine. 👍
2. I'm not able to get the actual syntax error location from something like <div></span> - I just get the stack with Module build failed. Using HMR and devtool: "eval". Is there further setup required?

[Restuta]

I am not using "webpack-dev-server" directly, I am using "webpack-dev-middleware", is there a way to have it as part of the middleware itself? @SpaceK33z

[karl]

Just tested this in our codebase at work and it behaved correctly as far as I could see. We would find this really useful for making it clear when compilation had failed.

We'd definitely use this if it was merged in!

[SpaceK33z]

@STRML, I just tested this in a project of mine with React and JSX. It looks like this when I change a </div> to </span>:

It didn't need any extra configuration. This was with devtool: "eval". I haven't setup HMR though (don't have the time right now to test that out).

@Restuta, no, webpack-dev-middleware can't communicate to the webpage since it's only middleware. webpack-hot-middleware does offer this functionality btw.

@karl, thanks for testing!

[Restuta]

@SpaceK33z I meant hot-middleware, sorry

[KutnerUri]

@SpaceK33z - is this still live? does it handle runtime errors?

I'm getting this different looking overlay