SocketErrorException is now a std::system_error: code() carries the errno
under std::generic_category(), or getaddrinfo's result under the new
ResolverCategory(), whose message() is gai_strerror's text; `phase` says
which step failed (resolve, open, connect, send, receive). It is still a
std::runtime_error with the message ClientTCP always threw, the code's
text after it, so existing handlers keep working.
Before, the exception carried a `kind` and an int, and a caller that had
to print the cause its own way still had to match the message prefix to
learn where the failure happened. The standard type ends both: the
category knows how to print the number, and the phase is a field. A
caller writes "[Errno {}] {}" from code().value() and code().message(),
or compares code() with std::errc::connection_refused.
Breaking: SocketErrorException's `kind` and `code` members are gone;
read code().category() and code().value().
Jorijn van der Graaf
e86c3b70a0