Unreachable-server UX: message boxes + bat guidance instead of a 'crash'

Field report (first internet-join attempt): both join bats 'hard
crash' on the remote machine.  Root cause was procedural -- the host
session was not running -- but the failure PRESENTATION was the bug:
Release Fail() is a bare abort(), so a dead relay = silent process
death + instantly-closing console window.

- L4NET: the two player-facing dead-ends (LAN discovery no-answer,
  seat request unreachable/full) now show a MessageBox saying what
  happened and what to do before exiting (verified live against a
  dead relay: 'BattleTech -- can't join the game' appears).
- join bats: echo + pause after exit so the window stays readable;
  join_lan.bat header now says it is LAN-only (the remote user ran
  both -- join_lan can never work over the internet).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tools/btoperator.py

@@ -706,6 +706,11 @@ class Operator(QMainWindow):
         # first-come-first-served (no BT_SELF), so every player gets the SAME
         # file -- who flies which mech is whoever sits down first, exactly
         # like walking up to a pod.
+        tail = ("echo.\n"
+                "echo The game has exited.  If you did NOT quit on purpose,\n"
+                "echo the session may not be running yet or the server was\n"
+                "echo unreachable -- check with the operator and try again.\n"
+                "pause\n")
         with open(os.path.join(out_dir, "join.bat"), "w", newline="\r\n") as f:
             f.write("@echo off\n"
                     "rem BT411 -- join %s's game (internet).  Your seat/mech\n"
@@ -713,16 +718,17 @@ class Operator(QMainWindow):
                     "set BT_RELAY=%s:%d\n"
                     "cd /d %%~dp0content\n"
                     "..\\build\\Release\\btl4.exe -egg %s -net 1501\n"
-                    % (host, host, self.f_port.value(), egg_name))
+                    % (host, host, self.f_port.value(), egg_name) + tail)
         with open(os.path.join(out_dir, "join_lan.bat"), "w",
                   newline="\r\n") as f:
             f.write("@echo off\n"
-                    "rem BT411 -- join the LAN game.  Finds the operator\n"
-                    "rem console AND your seat automatically.\n"
+                    "rem BT411 -- join the game from the operator's OWN\n"
+                    "rem network (LAN) -- finds the console automatically.\n"
+                    "rem Joining over the INTERNET?  Use join.bat instead.\n"
                     "set BT_RELAY=auto\n"
                     "cd /d %%~dp0content\n"
                     "..\\build\\Release\\btl4.exe -egg %s -net 1501\n"
-                    % egg_name)
+                    % egg_name + tail)
         self.log.appendPlainText(
             "exported join.bat + join_lan.bat to %s (public host: %s) -- "
             "one file for everyone; the relay assigns seats"