refs #159 - better error message in case getGlobal() failed

@@ -733,7 +733,9 @@ Casper.prototype.getGlobal = function getGlobal(name) {
         }
         return result;
     }, {'name': name});
-    if ('error' in result) {
+    if (typeof result !== "object") {
+        throw new CasperError(f('Could not retrieve global value for "%s"', name));
+    } else if ('error' in result) {
         throw new CasperError(result.error);
     } else if (utils.isString(result.value)) {
         return JSON.parse(result.value);