#2797: Clarify exception message, if type is not found in the module being decompiled.

ICSharpCode.Decompiler/CSharp/CSharpDecompiler.cs

@@ -953,7 +953,7 @@ namespace ICSharpCode.Decompiler.CSharp
 			if (type == null)
 				throw new InvalidOperationException($"Could not find type definition {fullTypeName} in type system.");
 			if (type.ParentModule != typeSystem.MainModule)
-				throw new NotSupportedException("Decompiling types that are not part of the main module is not supported.");
+				throw new NotSupportedException($"Type {fullTypeName} was not found in the module being decompiled, but only in {type.ParentModule.Name}");
 			var decompilationContext = new SimpleTypeResolveContext(typeSystem.MainModule);
 			var decompileRun = CreateDecompileRun();
 			syntaxTree = new SyntaxTree();