Fix return codes and messages

[ajoaoff]

Fixes #10 and fixes #9.

Description of the change

This PR changes the way error codes are returned, using werkzeug classes.
Also, fix the error where empty items were allowed

Tests were modified to reflect changes in code.

[ArturoQuintana]

We need to apply some changes to cover the same issues but focused on Patching

[ArturoQuintana]

The modifications to this method do not cover all the problems specified. For instance, patching the following payloads arises the same problem:

payload1 = {
            "start": start.strftime(TIME_FMT),
            "end": new_time.strftime(TIME_FMT),
            "items": []
        }

and

payload1 = {
            "start": start.strftime(TIME_FMT),
            "end": new_time.strftime(TIME_FMT)
        }

The solution resides in reusing the same approach used in create_mw. I have tested that adding the following code between lines 88 and 89 on main.py solve the issue:

try:            
    maintenance = MW.from_dict(data, self.controller)
except ValueError as err:            
    raise BadRequest(f'{err}')
if maintenance is None:
    raise BadRequest('One or more items are invalid')

[ajoaoff]

Doing it the way you suggested would create a new maintenance instance instead of updating.
The second payload is valid for updating, as you don't need to include all the arguments, just the ones you want to change.

[ArturoQuintana]

@ajoaoff after applying the patches, we have detected that the following payload is still throwing the same error (the return code is 200 instead of 400 as expected for an empty items field):

payload1 = {
            "start": start.strftime(TIME_FMT),
            "end": new_time.strftime(TIME_FMT),
            "items": []
        }