Don't fallback to PNSE, and give a more descriptive error message. (#88804)

Don't fallback to PNSE, and give a more descriptive error message. (#88804)

* Don't fallback to PNSE, and give a more descriptive error message.

* CodeGeneratorConversionException is internal. Wrap it before re-throwing.

* Update src/libraries/System.Private.Xml/src/System/Xml/Serialization/Compilation.cs

Fixing comment about lack of compiler at runtime

Co-authored-by: Jan Kotas <jkotas@microsoft.com>
* Update Compilation.cs

Remove conditional code that was not needed.

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Co-authored-by: Matt Connew <mconnew@users.noreply.github.com>
Co-authored-by: Jan Kotas <jkotas@microsoft.com>