pwquality: fix quality_check_password return value

pwquality: fix quality_check_password return value

quality_check_password() used to return the same value 0 in two
different cases: when pwq_allocate_context() failed with a
ERRNO_IS_NOT_SUPPORTED() code, and when pwquality_check() rejected the
password.  As result, users of quality_check_password() used to report
password weakness also in case when the underlying library was not
available.

Fix this by changing quality_check_password() to forward the
ERRNO_IS_NOT_SUPPORTED() code to its callers, and change the callers
to handle this case gracefully.