Change POST to /widgets to upload, parse and return error messages rather than just drop into /deploy

The current implementation of POST /widgets just drops a .wgt into /deploy and lets Wookie asynchronously handle the upload; this means that the uploader has no access to any error or success messages resulting from the operation.

Instead we need this method to operate in a similar manner to the admin servlet upload method so that a full range of response codes and messages can be returned to clients