http2.Error does not implement errors.Unwrap correctly

[pabigot]

#239 included addition of the errors package Unwrap() functionality to http2.Error, but the implementation does not fulfill the contract of that function, which is to return the wrapped error. Instead it returns a new untyped error containing only the text of the wrapped error.

This prevents applications from using Unwrap() (and so errors.As) correctly to get access to url.Error or net.Error instances that convey additional information about why the operation failed, making it more difficult to resolve #292.