builtin/show-ref: treat directory as non-existing in --exists

builtin/show-ref: treat directory as non-existing in --exists

9080a7f178 (builtin/show-ref: add new mode to check for reference
existence, 2023-10-31) added the option --exists to git-show-ref(1).

When you use this option against a ref that doesn't exist, but it is
a parent directory of an existing ref, you get the following error:

    $ git show-ref --exists refs/heads
    error: failed to look up reference: Is a directory

when the ref-files backend is in use.  To be more clear to user,
hide the error about having found a directory.  What matters to the
user is that the named ref does not exist.  Instead, print the same
error as when the ref was not found:

    error: reference does not exist

Signed-off-by: Toon Claes <toon@iotcl.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>