Print correct error message in utils/mkdir-p.pl

Print correct error message in utils/mkdir-p.pl

Commit 70a56b914772e6b21cda2a5742817ae4bb7290f1 introduced a regression.

If utils/mkdir-p.pl fails to create a target dir because of insufficient file system
permissions, the subsequent test for dir existence always fails and overwrites
the system error. As a result, a user is presented with a misleading error message.

E.g. if a user tries to create a dir under /usr/local and does not have permissions
for it, the reported error message is "Cannot create directory /usr/local/lib: No such file or directory",
whereas the expected error message is "Cannot create directory /usr/local/lib: Permission denied".

This commit introduces a fix by declaring an additional local variable to cache
the original error message from mkdir. If -d check fails and overwrites the system
error, the user is still presented with the original error from mkdir.

CLA: Trivial

Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/14487)