net/http: make "http: ContentLength=N with Body length 0" error message more intelligible

[leventov]

I think it would be nice to make error message more intelligible, e. g. "Request.Body's length (0) doesn't match Content-Length header (N)"

It may seem very obvious when you already know what the present error message means, but actually I couldn't understand. This SO question has 11k views and I suspect the majority of them are other confused developers.

[cagedmantis]

/cc @bradfitz @empijei @neild