Make empty selector message more helpful

[ysthakur]

Regarding #2601. Currently, if no arguments are given to mill, it says Selector cannot be empty. It's not immediately clear what that's supposed to mean, so I figured it might be more helpful to print the help message instead in that case (some other tools do that). In addition to this, I modified the empty selector message to say Selector cannot be empty. Try `mill resolve _` to see what's available. (the empty selector message still shows up if you do ./mill --repl).

[lefou]

I think showing a typical --help text without request isn't an expected user experience. If there is an issue, we should tell it the user. There is a large gap between some usage hint and an error message with details.

[lefou]

While reviewing this PR, I recognized that we left some traces of --repl suport in the code base. I opened #2611 to fix that.

[ysthakur]

Okay yeah, I guess an error message would be better than just usage instructions. Does Selector cannot be empty. Try `mill resolve _` to see what's available. look okay? Maybe instead of saying "selector," it could say you need a target? (since it's not immediately clear what a selector is)

[lefou]

Does Selector cannot be empty. Try `mill resolve _` to see what's available. look okay?

I think so.

Maybe instead of saying "selector," it could say you need a target? (since it's not immediately clear what a selector is)

How about "Target selector must not be empty ..."

[lefou]

Thank you!