Jayman2000 reports that users who do not update their MED.MNU after
updating to 8b61b9f ("Remove unused symbols") receive an "editor crash"
on startup. This is not a crash, but instead is poor error handling in
menubar.cpp, which calls UserError instead of ignoring the offending
line. Calls to UserError are fatal, but do not crash the program.
Replace that call with a con_printf(CON_URGENT) and ignore the line.
Additionally, change the handling for excess menus and for excess menu
items to con_printf+ignore instead of UserError.
Reported-by: Jayman2000 <https://github.com/dxx-rebirth/dxx-rebirth/pull/190>
Fixes: 8b61b9f0db ("Remove unused symbols")
1789643480