HAT: mark failed requests with an additional header

As we need to be sure we're catching all errors on hhvm and we are not
replaying more requests than we should on Zend, we need an easy way to
track that. The easiest path I found is to add an additional
X-Has-HHVM-Failures header in this case.

Change-Id: If406727960c9175c024f8fbf243de4d746b49b91
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>