BUG/MINOR: ssl: Fix error message after ssl_sock_load_ocsp call

BUG/MINOR: ssl: Fix error message after ssl_sock_load_ocsp call

If we were to enable 'ocsp-update' on a certificate that does not have
an OCSP URI, we would exit ssl_sock_load_ocsp with a negative error code
which would raise a misleading error message ("<cert> has an OCSP URI
and OCSP auto-update is set to 'on' ..."). This patch simply fixes the
error message but an error is still raised.

This issue was raised in GitHub #2432.
It can be backported up to branch 2.8.

(cherry picked from commit 73705ac701c6b0a8201ae29fffd9f29520a03b78)
Signed-off-by: Willy Tarreau <w@1wt.eu>