[FIX] barcodes: barcode handlers should return a deferred
[FIX] barcodes: barcode handlers should return a deferred

This was the case about 15minutes ago. But before pushing we
decided to change the Barcode override of FormController to
directly use handlers of the FormController when it could (e.g.
to handle 'Edit', 'Discard' actions). This wasn't a good idea as
those handlers don't return anything, and the barcode mechanism
requires its handlers to return a deferred. So basically, it
crashed when such a barcode was scanned. Having a test suite is
nice, but you should not forget to relaunch it before pushing...

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