Refs #482: fixes test onerror.js

Error message for a ReferenceError is not equal
between Gecko and Webkit.

@@ -2,12 +2,19 @@
 /*jshint strict:false*/
 casper.test.begin('page.error event tests', 2, function(test) {
     var error = {};
+    var expectedMessage;
+    if (phantom.casperEngine === 'phantomjs') {
+        expectedMessage = "ReferenceError: Can't find variable: plop";
+    }
+    else {
+        expectedMessage = "ReferenceError: plop is not defined";
+    }
     casper.once("page.error", function onError(msg, trace) {
         error.msg = msg;
         error.trace = trace;
     });
     casper.start('tests/site/error.html', function() {
-        test.assertEquals(error.msg, "ReferenceError: Can't find variable: plop",
+        test.assertEquals(error.msg, expectedMessage,
             "page.error event has been caught OK");
         test.assertMatch(error.trace[0].file, /error.html/,
             "page.error retrieves correct stack trace");