ArgumentParsingError for command not found should hint at the command that raised it

If the error class had an attribute with the command that raised it we could do more advanced error handling.

Right now I am looking into doing it this way:

```python
try:
cmd.run(...)
except ArgumentParsingError as err:
if err.__context__.args[0] not in dispatcher.commands.keys():
do_something()
else:
raise
```