Add innerException to ResourceReader FileNotFoundException (#72957)

Co-authored-by: Dan Moseley <[email protected]>

src/libraries/System.Private.CoreLib/src/System/Resources/ResourceReader.cs

@@ -971,6 +971,10 @@ private Type UseReflectionToGetType(int typeIndex)
                 Debug.Assert(_typeTable[typeIndex] != null, "Should have found a type!");
                 return _typeTable[typeIndex]!;
             }
+// If-defing this coud out from Resources Extensions since they will by definition always support deserialization
+// So we shouldn't attempt to wrap the original exception with a NotSupportedException since that can be misleading.
+// For that reason, the bellow code is only relevant when building CoreLib's ResourceReader.
+#if !RESOURCES_EXTENSIONS
             // If serialization isn't supported, we convert FileNotFoundException to
             // NotSupportedException for consistency with v2. This is a corner-case, but the
             // idea is that we want to give the user a more accurate error message. Even if
@@ -981,10 +985,12 @@ private Type UseReflectionToGetType(int typeIndex)
             //
             // We don't want to regress the expected case by checking the type info before
             // getting to Type.GetType -- this is costly with v1 resource formats.
-            catch (FileNotFoundException)
+            catch (FileNotFoundException fileNotFoundException) when (!_permitDeserialization)
             {
-                throw new NotSupportedException(SR.NotSupported_ResourceObjectSerialization);
+                // Include the FileNotFoundException as an inner exception to make it more diagnosable
+                throw new NotSupportedException(SR.NotSupported_ResourceObjectSerialization, fileNotFoundException);
             }
+#endif
             finally
             {
                 _store.BaseStream.Position = oldPos;