Improve missingCaseError error message

ts/util/missingCaseError.ts

@@ -1,6 +1,17 @@
 // Copyright 2018-2020 Signal Messenger, LLC
 // SPDX-License-Identifier: AGPL-3.0-only
 
+const stringify = (value: unknown): string => {
+  try {
+    // `JSON.stringify` can return `undefined` (TypeScript has incorrect types here).
+    //   However, this is fine because we interpolate it into a string, so it shows up as
+    //   "undefined" in the final error message.
+    return JSON.stringify(value);
+  } catch (err) {
+    return Object.prototype.toString.call(value);
+  }
+};
+
 // `missingCaseError` is useful for compile-time checking that all `case`s in
 // a `switch` statement have been handled, e.g.
 //
@@ -21,4 +32,4 @@
 // handled by our `switch` / `case` statement which is useful for code
 // maintenance and system evolution.
 export const missingCaseError = (x: never): TypeError =>
-  new TypeError(`Unhandled case: ${x}`);
+  new TypeError(`Unhandled case: ${stringify(x)}`);