Unhelpful error when trying to send a body on a GET request

[Demi-Marie]

#1926 was correct, but “channel closed” is not a helpful error message. Adding a new error that means “Request body not allowed” would be a significant help to users of the library.

[seanmonstar]

It would be quite difficult to get that error message in, since internally hyper simply drops the body, and the channel version doesn't have a way to know the difference.

If it doesn't already, we could include a log message about the ambiguity.

[Demi-Marie]

A log message might be a good idea.