dir: check afs_dir_MakeDir return code in dtest

dir: check afs_dir_MakeDir return code in dtest

The dtest test program ignores the return from afs_dir_Makedir.

Fix this so errors may be identified in testing.

While here, also improve the diagnostic message for afs_dir_Create
failures, to make it consistent with the new diagnostic message for
afs_dir_MakeDir failures.

Change-Id: Ib882947e01c864344f17faad8a646b2487793f29
Reviewed-on: https://gerrit.openafs.org/13797
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>