BUG/MEDIUM: ssl: crt-list must continue parsing on ERR_WARN

BUG/MEDIUM: ssl: crt-list must continue parsing on ERR_WARN

The original crt-list parsing was stopping at any non-zero value in
the cfgerr variable, even warnings.

This is an issue as it could lead to a crt-list parsing stopped at the
first warning, then HAProxy launched with a partial crt-list.

A ERR_WARN must continue the parsing. The parsing must be only
stopped on an ERR_CODE.

This commit is 2.1 only since it was fixed
in 2.2 by commit 2954c47 ("MEDIUM: ssl: allow crt-list caching")
and accidently in 2.0 by commit b131c87 ("CLEANUP: ssl: make
ssl_sock_load_cert*() return real error codes") as well as in 1.9 and
1.8.