aaa: handle credentials incorrect

CREDENTIALS_INVALID - user does not exist
CREDENTIALS_INCORRECT - password incorrect

engine does not have these two, so map them into the same.

Topic: AAA
Change-Id: I1e9c4edf6547f0483fd2484e575eeb762720eee2
Signed-off-by: Alon Bar-Lev <alonbl@redhat.com>