Fix the exception for unknown gdb cmd and add enhancement for monitor cmd

In gdbserver->handleMsg the else branch will return one variable to the caller, but the caller expects a list:
```
([resp, ack, detach] = self.handleMsg(data)) 
```
rather than one variable. So it will raise a value error here.
But in branch:
```
        if msg[1] == 'q':
            return self.handleQuery(msg[2:]), 1, 0
```
It's also possible for handleQuery to receive a unknown query cmd, but it will still return a list, and the gdb will output an unknown cmd imformation here. So I think it's better to keep the else branch be conformed with the behavior of handleQuery, and inform the user there's a unknown cmd is better than the exception of the program.