Replace string check with code check in test 'cms_sign_verify_error'.

        let res = cms.verify(None, Some(&empty_store), Some(data), None, CMSOptions::empty());

        // check verification result - this is an invalid signature

        // defined in openssl crypto/cms/cms.h

        const CMS_R_CERTIFICATE_VERIFY_ERROR: i32 = 100;

        match res {

            Err(es) => {

                let error_array = es.errors();

                assert_eq!(1, error_array.len());

                let err = error_array[0]

                    .code();

                assert_eq!(err, 0);

                let code = error_array[0].code();

                assert_eq!(ffi::ERR_GET_REASON(code), CMS_R_CERTIFICATE_VERIFY_ERROR);

            }

            _ => panic!("expected CMS verification error, got Ok()"),

        }