request #7012: improving REST error mesages on POST artifact

Change-Id: I239480b3b7c82922a8042c86087e7511b8a781ae
Merge commit 'refs/changes/92/2592/5' of ssh://gerrit.tuleap.net:29418/tuleap into stable

* ssh://gerrit.tuleap.net:29418/tuleap:
  request #7012: improving REST error mesages on POST artifact
This is Tuleap 7.1.99.42

Change-Id: Ib0c5e92d45792d530f53ec20c9067d8630298dc0