do not exit after first invalid file
do not exit after first invalid file

At the moment tools/validate.py exits after it found an
invalid file.

I think it's better to always validate all files and to
exit then if any invalid file was found. This way the
creator of a review request can fix all found issues in
all files and has not to create several patch sets.

Change-Id: Ic5668f0791680802b8cef3b5e1016c9dc76479b9

gongysh2004/openstack-manual,potsmaster/openstack-manuals,gongysh2004/openstack-manual,openstack/openstack-manuals,saeki-masaki/openstack-manuals,Akasurde/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,openstack/openstack-manuals,AlekhyaMallina-Vedams/openstack-manuals,kairoaraujo/openstack-manuals,kairoaraujo/openstack-manuals,Akasurde/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,saeki-masaki/openstack-manuals,potsmaster/openstack-manuals