use our _error stack to generate openpty errors

While we don't want these error messages on our usual stack, we can use
our usual infrastructure to generate an error message with all the usual
bells like errno and strerror attached.

Git-Dch: Ignore

apt-pkg/deb/dpkgpm.cc

@@ -1238,16 +1238,13 @@ bool pkgDPkgPM::Go(int OutStatusFd)
 
       // if tcgetattr does not return zero there was a error
       // and we do not do any pty magic
-      if (tcgetattr(0, &tt) == 0)
+      _error->PushToStack();
+      if (tcgetattr(STDOUT_FILENO, &tt) == 0)
       {
 	 ioctl(0, TIOCGWINSZ, (char *)&win);
-	 if (openpty(&master, &slave, NULL, &tt, &win) < 0) 
+	 if (openpty(&master, &slave, NULL, &tt, &win) < 0)
 	 {
-	    const char *s = _("Can not write log, openpty() "
-	                      "failed (/dev/pts not mounted?)\n");
-	    fprintf(stderr, "%s",s);
-            if(d->term_out)
-              fprintf(d->term_out, "%s",s);
+	    _error->Errno("openpty", _("Can not write log (%s)"), _("Is /dev/pts mounted?"));
 	    master = slave = -1;
 	 }  else {
 	    struct termios rtt;
@@ -1265,6 +1262,15 @@ bool pkgDPkgPM::Go(int OutStatusFd)
 	    sigprocmask(SIG_SETMASK, &original_sigmask, 0);
 	 }
       }
+      // complain only if stdout is either a terminal (but still failed) or is an invalid
+      // descriptor otherwise we would complain about redirection to e.g. /dev/null as well.
+      else if (isatty(STDOUT_FILENO) == 1 || errno == EBADF)
+         _error->Errno("tcgetattr", _("Can not write log (%s)"), _("Is stdout a terminal?"));
+
+      if (_error->PendingError() == true)
+	 _error->DumpErrors(std::cerr);
+      _error->RevertToStack();
+
        // Fork dpkg
       pid_t Child;
       _config->Set("APT::Keep-Fds::",fd[1]);