assert: fix generatedMessage property

This makes sure the `generatedMessage` property is always set as
expected. This was not the case some `assert.throws` and
`assert.rejects` calls.

Backport-PR-URL: #31431
PR-URL: #28263
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/assert.js

@@ -578,14 +578,21 @@ function expectedException(actual, expected, message, fn) {
       if (expected.test(str))
         return;
 
-      throw new AssertionError({
+      if (!message) {
+        generatedMessage = true;
+        message = 'The input did not match the regular expression ' +
+                  `${inspect(expected)}. Input:\n\n${inspect(str)}\n`;
+      }
+
+      const err = new AssertionError({
         actual,
         expected,
-        message: message || 'The input did not match the regular expression ' +
-                            `${inspect(expected)}. Input:\n\n${inspect(str)}\n`,
+        message,
         operator: fn.name,
         stackStartFn: fn
       });
+      err.generatedMessage = generatedMessage;
+      throw err;
     }
 
     // Handle primitives properly.