Relax error check

The precise error is version-dependent, just check that there
is some kind of error reported.

Author: nikic

ext/openssl/tests/bug80747.phpt

@@ -15,9 +15,9 @@ $conf = array(
     'private_key_bits' => 511,
 );
 var_dump(openssl_pkey_new($conf));
-echo openssl_error_string(), "\n";
+var_dump(openssl_error_string() !== false);
 
 ?>
---EXPECTF--
+--EXPECT--
 bool(false)
-error:%s:key size too small
+bool(true)