Improve error message when misspelling a XAML namespace

[ashelleyPurdue]

I just spent 2 hours trying to figure out why my XAML wouldn't compile. Turns out I had added written "https" instead of "http" in the url for the "x" namespace. The error message I got was this:

Avalonia error AVLN:0004: Unable to find suitable setter or adder for property ThemeDictionaries of type Avalonia.Base:Avalonia.Controls.ResourceDictionary for argument Avaloni
a.Base:Avalonia.Controls.ResourceDictionary, available setter parameter lists are:

...which led me to believe my mistake was somewhere in my usage of ThemeDictionaries.

A much more helpful error message would have been something like this:

Namespace "https://schemas.microsoft.com/winfx/2006/xaml" does not exist.

Not sure if that's possible, but I can dream, can't I?