Error messages for most confusing scenarios

Impact

• Errors not only show that there is a problem, they help the user to fix it and to learn more about Rust (possibly directing the user to other documentation).

Design notes

Of course there are an infinite number of improvements one could make. The point of this deliverable is to target the most common situations and confusions people see in practice. The final list is still being enumerated:

• Confusing error: Immutable reference to future is not a future rust-lang/rust#87211