Show XAML errors separately

[ForNeVeR]

We have hidden XAML errors in #203, but now it's problematic to figure out what are the real errors (e.g. see #259).

I propose let's do two things:

1. Whenever there's a XAML error in previewer, show a red indicator on the toolbar.
2. When clicking that toolbar button, expose the error text in a copy-pastable way.

[nielsbosma]

This would be a great fix. If there's an issue in the ViewModel it's very difficult to find it.