Ensure the correct error message is displayed

When a untranslated error message pass through the translation pecan
hook, the previous translated error is send to the client.

This patch fix that by reseting the "translatable_error" of the
translation pecan hook before each request.

Conflicts:

    tests/api/v2/test_app.py

Change-Id: Icee79cf09fd696655048c7c0cf8a3d6706e31701
Fixes: bug #1260398
(cherry picked from commit 02d017022bab577b581b58f42b468bf0b2c644a9)
Merge "Ensure the correct error message is displayed" into stable/havana