flux-fsck: fix error message output

Problem: An error check inadvertently checks `errno` when it
should check the stored errnum in a data structure.  This leads
to an invalid error message output.

Fix the error check to ensure the correct error message is output.

Fixes #6860

Author: chu11

src/cmd/builtin/fsck.c

@@ -147,7 +147,7 @@ static void fsck_valref (flux_t *h, const char *path, json_t *treeobj)
     if (fvd.errorcount) {
         /* each invalid blobref will be output in verbose mode */
         if (!verbose) {
-            if (errno == ENOENT)
+            if (fvd.errnum == ENOENT)
                 read_error ("%s: missing blobref(s)", path);
             else
                 read_error ("%s: error retrieving blobref(s): %s",