Nicer error when running tmux exploit outside tmux (#2314)

When you set the `context.terminal` to be tmux related but then try
to run the exploit outside a tmux session, you'd get a cryptic ValueError
while trying to parse the pid of the new pane in tmux.

Print a nicer reminder to start tmux first.

pwnlib/util/misc.py

@@ -410,7 +410,12 @@ def run_in_new_terminal(command, terminal=None, args=None, kill_at_exit=True, pr
 
     if terminal == 'tmux':
         out, _ = p.communicate()
-        pid = int(out)
+        try:
+            pid = int(out)
+        except ValueError:
+            pid = None
+        if pid is None:
+            log.error("Could not parse PID from tmux output (%r). Start tmux first.", out)
     elif terminal == 'qdbus':
         with subprocess.Popen((qdbus, konsole_dbus_service, '/Sessions/{}'.format(last_konsole_session),
                                'org.kde.konsole.Session.processId'), stdout=subprocess.PIPE) as proc: