Refactor: libpe_status: Replace pcmk_rc_schema_validation uses

...with more appropriate return codes -- namely EINVAL for a bad
argument (developer mistake) or pcmk_rc_unpack_error for a failure in
parsing the CIB. pcmk_rc_schema_validation is more properly used for an
update that doesn't conform to the configured schema.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>