Harness: Account for Symbols being thrown in async tests

It's possible for an async test to throw a Symbol (harness/async-gc.js
does this.) The Symbol ends up getting passed to $DONE in a
.then($DONE, $DONE) call. Previously, $DONE would then throw an exception
due to not being able to convert the Symbol to a string.

harness/doneprintHandle.js

@@ -14,7 +14,7 @@ function $DONE(error) {
     if(typeof error === 'object' && error !== null && 'name' in error) {
       __consolePrintHandle__('Test262:AsyncTestFailure:' + error.name + ': ' + error.message);
     } else {
-      __consolePrintHandle__('Test262:AsyncTestFailure:Test262Error: ' + error);
+      __consolePrintHandle__('Test262:AsyncTestFailure:Test262Error: ' + String(error));
     }
   } else {
     __consolePrintHandle__('Test262:AsyncTestComplete');