errors: add error 59, for unknown command.

Author: Christian Hergert

mongoc/mongoc-error.h

@@ -59,6 +59,7 @@ typedef enum
 
    MONGOC_ERROR_QUERY_FAILURE,
 
+   MONGOC_ERROR_QUERY_COMMAND_NOT_FOUND = 59,
    MONGOC_ERROR_QUERY_NOT_TAILABLE = 13051,
 } mongoc_error_code_t;