Fix SLA #5876 : There is no error message on FRS form validation

* On IE, the form seems to be submited several times.
  To fix it, I stop the event before the Ajax request is sent to Tuleap

Change-Id: I0a39c8b373ec863f25ed4542d51704ae478a09e3
Merge commit 'refs/changes/35/1035/1' of ssh://gerrit.tuleap.net:29418/tuleap into tuleap-stable-master

* ssh://gerrit.tuleap.net:29418/tuleap:
  Fix SLA #5876 : There is no error message on FRS form validation

Change-Id: Iec0522514c4888f550faa03d5f08b632a8fa375e
Tuleap 6.5.99.26

Change-Id: I369376c9a76a8b4a08364f4ee5f4e4424a33bc7e