Warn the user if they try to assign someone without review privileges

[varkor]

Currently, if you r? a user who doesn't have the right to r+, rust-highfive will silently ignore it. This can be a little confusing: it'd be nicer if there was some indication of what went wrong.

[lcnr]

Did this change or am I misinterpreting this issue 😅 I've gotten assign to a PR without having the right to r+ in rust-lang/rust#73254

[varkor]

Huh, it does seem to work now. I'll close unless there's any indication otherwise.