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: NJan Kotas <jkotas@microsoft.com>

* Update Compilation.cs

Remove conditional code that was not needed.

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

src/libraries/System.Private.Xml/src/Resources/Strings.resx

@@ -3426,6 +3426,9 @@ Usage: dotnet {0} [--assembly <assembly file path>] [--type <type name&
   <data name="GenerateSerializerNotFound" xml:space="preserve">
     <value>Method 'System.Xml.Serialization.XmlSerializer.GenerateSerializer' was not found. This is likely because you are using an older version of the framework. Please update to .NET Core v2.1 or later.</value>
   </data>
+  <data name="CodeGenConvertError" xml:space="preserve">
+    <value>CodeGenError({0}): Cannot convert source type [{1}] to target type [{2}].</value>
+  </data>
   <data name="CompilingScriptsNotSupported" xml:space="preserve">
     <value>Compiling JScript/CSharp scripts is not supported</value>
   </data>

src/libraries/System.Private.Xml/src/System/Xml/Serialization/CodeGenerator.cs

@@ -1628,7 +1628,7 @@ internal sealed class CodeGeneratorConversionException : Exception
         private readonly string _reason;
 
         public CodeGeneratorConversionException(Type sourceType, Type targetType, bool isAddress, string reason)
-            : base()
+            : base(SR.Format(SR.CodeGenConvertError, reason, sourceType.ToString(), targetType.ToString()))
         {
             _sourceType = sourceType;
             _targetType = targetType;

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

@@ -59,9 +59,6 @@ internal TempAssembly(XmlMapping[] xmlMappings, Type?[] types, string? defaultNa
                 }
             }
 
-            // We will make best effort to use RefEmit for assembly generation
-            bool fallbackToCSharpAssemblyGeneration = false;
-
             if (!containsSoapMapping && !TempAssembly.UseLegacySerializerGeneration)
             {
                 try
@@ -69,19 +66,18 @@ internal TempAssembly(XmlMapping[] xmlMappings, Type?[] types, string? defaultNa
                     _assembly = GenerateRefEmitAssembly(xmlMappings, types);
                 }
                 // Only catch and handle known failures with RefEmit
-                catch (CodeGeneratorConversionException)
+                catch (CodeGeneratorConversionException ex)
                 {
-                    fallbackToCSharpAssemblyGeneration = true;
+                    // There is no CSharp-generating/compiling fallback in .Net Core because compilers are not part of the runtime.
+                    // Instead of throwing a PNSE as a result of trying this "fallback" which doesn't exist, lets just let the
+                    // original error bubble up.
+                    //fallbackToCSharpAssemblyGeneration = true;
+                    throw new InvalidOperationException(ex.Message, ex);
                 }
                 // Add other known exceptions here...
                 //
             }
             else
-            {
-                fallbackToCSharpAssemblyGeneration = true;
-            }
-
-            if (fallbackToCSharpAssemblyGeneration)
             {
                 throw new PlatformNotSupportedException(SR.CompilingScriptsNotSupported);
             }