flux-overlay: show mismatched versions in errors

Problem: 'flux overlay errors' does not display mismatched version
errors because it only considers lost nodes.

Show any errors recorded for offline nodes too.

Update sharness test to expect these errors.

Fixes #6199

Author: garlick

src/cmd/builtin/overlay.c

@@ -913,8 +913,11 @@ void gather_errors (flux_t *h,
                 log_msg_exit ("error adding to error hash");
         }
         else if (streq (status, "offline")) {
-            /* Don't report offline nodes.
-             */
+            // report offline only if there is error text
+            if (error && strlen (error) > 0) {
+                if (errhash_add_one (errhash, child_rank, error) < 0)
+                    log_msg_exit ("error adding to error hash");
+            }
         }
         else { // recurse
             gather_errors (h, child_rank, errhash, timeout);

t/t3303-system-healthcheck.t

@@ -139,9 +139,9 @@ test_expect_success 'flux overlay status shows rank 3 offline' '
 	test_cmp health.exp health.out
 '
 
-test_expect_success 'flux overlay errors prints nothing' '
+test_expect_success 'flux overlay errors shows rank 3' '
 	flux overlay errors --timeout=0 >errors2.out &&
-	test $(wc -l <errors2.out) -eq 0
+	grep "fake3: administrative shutdown" errors2.out
 '
 
 test_expect_success 'flux overlay status --summary' '
@@ -202,9 +202,8 @@ test_expect_success 'ping to rank 14 fails with EHOSTUNREACH' '
 '
 
 test_expect_success 'flux overlay errors shows the lost connection' '
-	echo "fake14: lost connection" >errors3.exp &&
 	flux overlay errors --timeout=0 >errors3.out &&
-	test_cmp errors3.exp errors3.out
+	grep "fake14: lost connection" errors3.out
 '
 
 test_expect_success 'wait for rank 0 subtree to be degraded' '