usability: error if extra arguments passed to --all

[kevinburke]

I wanted to delete two dependencies. I tried gb vendor delete a b and got:

FATAL: command "delete" failed: delete: import path or --all flag is missing

I then tried:

gb vendor delete --all a b

I expected that this would delete both a and b, but instead deleted all packages.

I think --all should error if any positional arguments are passed, and/or delete could have a clearer error message if multiple positional arguments are passed.