Harmonize response type

Here we discuss of the new responses

When it works : For get_label:

• label : main label in pdf or zpl or or url
• label_type: ?
• barcode ?
• tracking number ?
• tracking url ?
• annexes: aknowledge or CN23 or something else { document: pdf or zpl or ... document_type: ? document_title: ? }

All requests:

• success: True | False

When it doesn't works : What may fail:

• Validation error (before request)
• Network error
• WS error
• Other

• success: False
• messages: [ { type: ?, message: text, id: ?, } ] Raise exceptions or error ?