MINOR: ssl: ssl_sock_prepare_ctx() return an error code

Rework ssl_sock_prepare_ctx() so it fills a buffer with the error
messages instead of using ha_alert()/ha_warning(). Also returns an error
code (ERR_*) instead of the number of errors.
diff --git a/include/proto/ssl_sock.h b/include/proto/ssl_sock.h
index d6d01a7..689fbab 100644
--- a/include/proto/ssl_sock.h
+++ b/include/proto/ssl_sock.h
@@ -45,7 +45,7 @@
 		return 1;
 }
 
-int ssl_sock_prepare_ctx(struct bind_conf *bind_conf, struct ssl_bind_conf *, SSL_CTX *ctx);
+int ssl_sock_prepare_ctx(struct bind_conf *bind_conf, struct ssl_bind_conf *, SSL_CTX *ctx, char **err);
 int ssl_sock_prepare_all_ctx(struct bind_conf *bind_conf);
 int ssl_sock_prepare_bind_conf(struct bind_conf *bind_conf);
 int ssl_sock_prepare_srv_ctx(struct server *srv);