Awesome error messages

[trieloff]

Let's make sure the error messages shown in the browser during development are really good.

I'd like to see:

• request dump (all headers and cookies)
• full stack trace
• code preview for all project code (resolve source maps, too)
• log messages from current request

Edit: something like Whoops would be ideal.

[trieloff]

Here's the other issue @kamendola

[trieloff]

adobe/helix-pipeline#61 will be useful here

[rofe]

@kamendola can you add or link to the mockups?