Bug #42226 ALTER DATABASE returns OK even when there are errors

Description:
ALTER DATABASE, within mysql_alter_db(), will return OK, even if there were errors, such as writing to the binary logs, or getting global read locks etc. 

How to repeat:
N/A - Code Review

Suggested fix:
Return errors when there are errors, so that the client does not think everything went OK.

Specifically, from mysql_alter_db() in sql/sql_db.cc:

  build_table_filename(path, sizeof(path), db, "", MY_DB_OPT_FILE, 0);
  if ((error=write_db_opt(thd, path, create_info)))
    goto exit;
...
exit:
  VOID(pthread_mutex_unlock(&LOCK_mysql_create_db));
  start_waiting_global_read_lock(thd);
exit2:
  DBUG_RETURN(error);