Add 'error' property to exceptions thrown via Worker::importScripts.

LayoutTests/http/tests/workers/worker-importScripts-onerror-sameorigin.html

Index: LayoutTests/http/tests/workers/worker-importScripts-onerror-sameorigin.html
diff --git a/LayoutTests/http/tests/workers/worker-importScripts-onerror-sameorigin.html b/LayoutTests/http/tests/workers/worker-importScripts-onerror-sameorigin.html
index b3a2ef35654c5957029e25bf910fbe4154f77fcb..ef13fd1f86f17120bf4ac12ba53708770afa8ebf 100644
--- a/LayoutTests/http/tests/workers/worker-importScripts-onerror-sameorigin.html
+++ b/LayoutTests/http/tests/workers/worker-importScripts-onerror-sameorigin.html
@@ -21,7 +21,8 @@
             shouldBeEqualToString("workerOnerror.filename", "http://127.0.0.1:8000/workers/resources/worker-importScripts-throw.js");
             shouldEvaluateTo("workerOnerror.lineno", 1);
             shouldEvaluateTo("workerOnerror.colno", 0);
-            shouldBeNull("workerOnerror.error");
+            shouldNotBe("workerOnerror.error", "null");
+            shouldBeEqualToString("workerOnerror.error", "This is a custom error message.");
         };
 
         var onerrorMessage, onerrorURL, onerrorLine;