fix: use isError instead of instanceof in innerOk to define message type

Co-Authored-By: Ruben Bridgewater <ruben@bridgewater.de>
Co-Authored-By: Nihar Phansalkar <phansalkarnihar@gmail.com>

lib/assert.js

@@ -393,7 +393,7 @@ function innerOk(fn, argLen, value, message) {
     } else if (message == null) {
       generatedMessage = true;
       message = getErrMessage(message, fn);
-    } else if (message instanceof Error) {
+    } else if (isError(message)) {
       throw message;
     }
 

test/parallel/test-assert.js

@@ -55,6 +55,16 @@ assert.throws(() => a.ok(false), a.AssertionError, 'ok(false)');
   assert.ok(threw, 'Error: ok(false)');
 }
 
+// Errors created in different contexts are handled as any other custom error
+{
+  const context = vm.createContext();
+  const error = vm.runInContext('new SyntaxError("custom error")', context);
+
+  assert.throws(() => assert(false, error), {
+    message: 'custom error',
+    name: 'SyntaxError'
+  });
+}
 
 a(true);
 a('test', 'ok(\'test\')');