Assembly name in XML namespace definition must be case-sensitive, otherwise getting NotImplementedException

* .NET Core Version: (e.g. 3.0 Preview1, or daily build number, use `dotnet --info`)
Bug found in .NET Framework 4.7.2, however it may happen as well in .NET Core

* Windows version: (`winver`)
Windows 10 version 18363.1734

* Does the bug reproduce also in WPF for .NET Framework 4.8?: Yes/No
Unknown

* Is this bug related specifically to tooling in Visual Studio (e.g. XAML Designer, Code editing, etc...)? If yes, please file the issue via the instructions [here](https://docs.microsoft.com/visualstudio/ide/how-to-report-a-problem-with-visual-studio?view=vs-2019).
No

**Problem description:**

When there's a mismatch in assembly name casing in XML namespace definition, `NotImplementedException` is thrown without any useful information on what went wrong.

The actual exception is thrown in Baml2006SchemaContext.cs, `ResolveBamlType`.

**Actual behavior:**

`NotImplementedException` is thrown.

**Expected behavior:**

If assembly name in XML namespace definition is case-sensitive, that's fine, but then:
* Code Insight should probably point that out as an error (it reacts properly if the name is completely wrong)
* Exception message should be more insightful, eg. "Cannot find assembly with name MsLib defined in namespace xyz"

**Minimal repro:**

To reproduce:
* Create solution with two projects: WPF application and a class library
* Create a view in WPF application
* Add namespace declaration pointing out to the second project and misspell the assembly by the casing (eg. MsLib instead of MSLib)
* Use that namespace (eg. put converter in there or a user control)

It may be important (or not), that in my case the problem happened in Windows Forms application, which used `ElementHost` to display WPF content inside WinForms window. In that case the application should be WF instead of WPF and should host a custom WPF control, but other than that, repro steps doesn't differ.