--watch flag not always recognized #798
Description
The --watch cli flag is only recognized when the watch extra is installed, leading to user confusion.
e.g. #702, Bogdanp/django_dramatiq#193, Bogdanp/django_dramatiq#91, Bogdanp/django_dramatiq#161
We should change it so the --watch flag is always recognised as an argument, but using it without watch installed, prints a useful error about what to install, rather than the current generic error.