JSON store should provide mechanism to return error message when creating and updating records

The ExtJsonStore's createRecord and updateRecord methods should provide a convenient way to return an error to the client when creating or updating fails for some reason.

Currently, both methods either return the created/updated record as a JSON object, or throw a JSON exception (which leads to a WicketRuntimeException). The response can be customized by overriding createResponse(), but this is decoupled from the createRecord/updateRecord method calls.

Instead, the create/update methods should be able to throw an exception, which automatically leads to an error response.