Improve error message if handle() method not implemented in Command.

[brettp]

Small quality of life improvement that prevents an unhelpful error if you have a typo in the handle() method (and spent longer than you should have wondering what wasn't working).

Was:
ERROR:root:[TestCommand] Error:

Now:
ERROR:root:[TestCommand] Error: handle() method not implemented

[brettp]

I updated this to improve the error logging for the other case also by adding the exception class.

Was:

ERROR:root:[TestCommand] Error: 'res'

Now:

ERROR:root:[TestCommand] <class 'KeyError'> Error: 'res'

[filipre]

To make the message nicer, can we use e.__class__.__name__ instead? What about this format:

f"[{command.__class__.__name__}] {e.__class__.__name__}: {e}"