Fix error handling in compiler watch() and run()

[SpaceK33z]

What kind of change does this PR introduce?

Bugfix

Did you add tests for your changes?

Yes.

Summary

See #156. When the compiler wanted to report an error in the bundle (e.g. because of a plugin), it would throw the error and quit the process.

I've changed this to work more like the webpack watch mode; it shows the error with console.error but it keeps running (because the error can be fixed with a new compilation).

Optionally, it's possible to hide / customize this error by using options.error.

Does this PR introduce a breaking change?

Not really; previously the process would quit and now it doesn't.

Other information

Fixes #156

[hansl]

Never mind my last message.

This PR works, partially. The error message shows up in the console properly, but the compilation still hangs. In this example I throw new Error('hello world'):

🏠  ng serve
** NG Live Development Server is running on http://localhost:4200. **
 10% building modules 2/2 modules 0 activeError: hello world
    at /Users/hans/Sources/angular-cli/packages/@ngtools/webpack/src/plugin.ts:235:19
    at process._tickCallback (internal/process/next_tick.js:103:7)
 69% building modules 678/679 modules 1 active ...ns/Sources/_Temp/my-project/src asyncwebpack: wait until bundle finished: /worker.js

[hansl]

LGTM.

[SpaceK33z]

Thanks for verifying @hansl.