Log system does not handle errors

I have a samsung duo something and the command "input tap" does not exists.
The log window didn't tell about it I had to manually check the code and repeat the operation.

```
$ adb shell input tap 0 0
Error: Unknown command: tap
usage: input [text|keyevent]
       input text <string>
       input keyevent <event_code>
```

Could be nice to add at least an error message.
