Log %err_code for ERR_RELAY_REMOTE transactions (#1472)

Log %err_code for ERR_RELAY_REMOTE transactions (#1472)

For ERR_RELAY_REMOTE transactions, Squid was logging %err_code as "-"
because BuildHttpReply() was not updating HttpRequest::error or ALE.
That update was missing because the pre-computed response in those
transactions triggered a premature exit from BuildHttpReply().

BuildHttpReply() should not be updating errors at all, but significant
code refactoring required to fix that problem needs a dedicated change.

Also enabled regression testing for the fixed bug and the bug fixed in
recent commit ea3f56e.