fix(js/ts): make Exception.ToString() return the message instead of "Exception" (#4635)

Co-authored-by: Kristofer Löfberg <kristofer.lofberg@gmail.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: 3 people

src/fable-library-ts/Util.ts

@@ -84,6 +84,10 @@ export class Exception {
   constructor(msg?: string) {
     this.message = msg ?? "";
   }
+
+  toString() {
+    return this.message;
+  }
 }
 
 export function isException(x: any) {

tests/Js/Main/TypeTests.fs

@@ -1393,6 +1393,15 @@ let tests =
         | ex -> (false, "unknown", 0.)
         |> equal (true, "Code: 5", 5.5)
 
+    testCase "Exception ToString contains the message" <| fun () -> // See #4197
+        let msg =
+            try
+                failwith "test message"
+                ""
+            with ex -> ex.ToString()
+
+        msg.Contains("test message") |> equal true
+
     testCase "reraise works" <| fun () ->
         try
             try