No longer reading the URL parameter if the config specifies a main module

This pull request changes the order in which configurations are merged. If a main module is already defined (either in the packaged configuration, or in the script tag configuration), the URL parameter is now ignored (i.e. what's after the question mark is not interpreted as the main module).
Said in another way, this means that the URL parameter is now interpreted as the main module only if the configuration (after merging both the packaged one or the one from the script tag) does not specify a main module.
This solves an issue in the [integration of noder-js with Aria Templates](http://ariatemplates.com/blog/2014/05/noder-js-integration-in-aria-templates/) when using the [Aria Templates CDN](http://cdn.ariatemplates.com):
> Note that if you use a question mark to pass parameters to the CDN (for example dev or skin), they are currently also interpreted by noder-js as the main file to be executed (instead of the default aria/bootstrap file).

This pull request also contains a small unrelated commit which changes the way the error handler is called (to be more intuitive).