Show errors returned from sourceRequest

[weinand]

Today errors returned from the sourceRequest are only visible on the dev console.

For that reason the sourceRequest implementation of node-debug manufactures content that contains the error message and returns success.

I've cleaned that up and now node-debug returns proper errors.

For the UI we have these option:

1. workbench error notification
2. manufacturing an editor content for the error and showing this in the editor
3. having a '404'-like error page instead misusing an editor for that.

Option 1 is too heavy because the user has to dismiss the notification.
Option 2 is what we have today but implemented on the 'correct' side.
Option 3 would be the best solution.

For now 2 is fine by me, but we should involve UX to design something for 3.

[weinand]

@isidorn this is how the error messages looks like:

Especially ugly is the line number '1' for the error message and that the execution line and the arrow is shown.

(This is meant as input to the UX design meeting)