BUG/MEDIUM: ssl: error when no certificate are found

BUG/MEDIUM: ssl: error when no certificate are found

When a non-existing file was specified in the configuration, haproxy
does not exits with an error which is not normal.

This bug was introduced by dfa93be ("MEDIUM: ssl: emulate multi-cert
bundles loading in standard loading") which does nothing if the stat
failed.

This patch introduce a "found" variable which is checked at the end of
the function so we exit with an error if no find were found.

Must be backported to 2.3.

(cherry picked from commit 06ce84a10079388e95e5959dbcb50fcc563c702b)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>