Improved error reporting for corrupted master page

[acizmarik]

This PR fixes weird DotVVM error reported by @martindybal. If user makes a mistake while specifying viewmodel (by referencing a non-existing class) for their master page, they receive a somewhat confusing error message.

The problem:

• DotVVM finds out that the specified viewmodel could not be found

  dotvvm/src/DotVVM.Framework/Compilation/ControlTree/Resolved/ResolvedTreeBuilder.cs

  Line 149 in 7dc7e9d

  directive.AddError($"Could not resolve type '{nameSyntax.ToDisplayString()}'.");

• However this information gets stripped and is not propagated further

  dotvvm/src/DotVVM.Framework/Compilation/ControlTree/Resolved/ResolvedTreeRoot.cs

  Line 29 in 7dc7e9d

  select (ds.Key, d.Value)).ToImmutableArray(),

If we were to add those errors there, I am not sure whether it could break something. In the meantime, I created a simple fix that should not break anything and provides users with a more specific error message.

[exyi]

Ok.