Report error when menu() can't find a display file and P_NOERROR not used

This issue was introduced in commit d02fc1a2 (3 years ago), where menu() would
silenty fail (just return false) if no display file matching the supported
menu/display file types/extensions could be found.

Previous to commit d02fc1a2, we would log a NOTICE-level message and display
a "File not found" message to the user (what sbbs_t::printfile() does), but
since d02fc1a2, we no longer even call printfile() when the file doesn't
exist, so no error was logged or message displayed to the user.

Of course, when using the P_NOERROR mode flag, silent failure is expected and
that's still what happens in that case.