broker: fix cert_create() error handling

Problem: in overlay_create(), all cert_create() failures are
reported as a generic ENOMEM, without calling out cert_create().

Since cert_create() sets errno, allow errno to propagate on failure.
Also add a log message indicating that cert_create() is the cause
of the failure since overlay_create() is a long function.

This problem was noted in #6151.

Author: garlick

src/broker/overlay.c

@@ -2493,8 +2493,10 @@ struct overlay *overlay_create (flux_t *h,
         goto error;
     if (flux_msg_handler_addvec (h, htab, ov, &ov->handlers) < 0)
         goto error;
-    if (!(ov->cert = cert_create ()))
-        goto nomem;
+    if (!(ov->cert = cert_create ())) {
+        log_err ("could not create curve certificate");
+        goto error;
+    }
     if (!(ov->health_requests = flux_msglist_create ())
         || !(ov->trace_requests = flux_msglist_create ()))
         goto error;