This was introduced in fa125b9, and
then "reverted" in 1cf4801, except that
revert left the struct around doing nothing useful.

We're removing it all the way now because we want to make a new
`TeeSink` complementing the already-exiting `TeeSource`, that is
actually a completely different concept as far as the class hierarchy is
concerned.

This is a bit complex because we want to expose extra functionality the
wrapped class has. Perhaps there is some inheritancy trickery to do this
nicer, but I don't know it, and this is the first thing we tried after a
series of attempts that did build.

This design is kind of like that of Rust's Writer, Reader, or Iter
adapters, which impliment more traits based on what the inner type
implements.

Everywhere seems to use `getHTTPStatus` now.

We have a larger problem that passsing computed strings to the first
variable argument of many exception constructors is unsafe because that
first variable argument is interpreted not as a plain string, but format
string, and if it contains '%' boost::format will abort, since there are
no arguments to the format string.

In this particular instance '%' was used as part of an escape code in a
URL, which, when the download failed, caused Nix to abort displaying the
`FileTransferError`.

…to-network-errors

 - Bad dynamic cast target ...classic

 - std::shared_ptr need explicit deref

…rk-errors

…k-errors

…k-errors

Due to #3841 we don't know how print
different messages for different verbosity levels.

…rk-errors

Add response body to network errors