ParseConfig: default_query_exec_mode: Return arg in error

If the default_query_exec_mode is unknown, the returned error
previously was:

    invalid default_query_exec_mode: <nil>

This changes it to return the argument. Add a test that unknown modes
fail to parse and include this string.

conn.go

@@ -178,7 +178,7 @@ func ParseConfigWithOptions(connString string, options ParseConfigOptions) (*Con
 		case "simple_protocol":
 			defaultQueryExecMode = QueryExecModeSimpleProtocol
 		default:
-			return nil, fmt.Errorf("invalid default_query_exec_mode: %v", err)
+			return nil, fmt.Errorf("invalid default_query_exec_mode: %s", s)
 		}
 	}