forget to name the argument

Now when we try to update an object that do not exists a 404 is answered and not a 500 anymore

Now all "empty" values are deleted from the resulting array

Implements a new exception class for webui : HttpErrors that uses a new template : errors.html to display multiples errors

- updated scripts etc
- make tests pass

- Replaced some "hidden" import with LodelContext.expose_modules calls
 - in some functions of Setting, Plugin, validator
 - in loader.py (temporary version)

In this state the tests runs (151 success), I'm able to run an instance (from debian package using slim) in MONOSITE mode and in MULTISITE mode by creating and loading a context in the loader.

Theorically all of those imports were tested by unit testing, but we've got no inssurance about that.
And even if unit tests had check for syntax errors, all pieces of code were not tested. We cannot
be sure that an import was missed or forgotten...