Better failure mode for invalid root-menu.plist

[rjhorniii]

The present failure mode for an invalid root-menu.plist is to exit with a parser error message to stderr. This is difficult for the new user. There is code for a default root-menu if no root-menu file can be found. I suggest a default alternative root-menu to use when the provided root-menu is invalid. Something short and simple like a title "Invalid root menu file" with one entry that exits wlmaker cleanly would be enough. Putting in the text from the parser error would be helpful but might not fit the space available. Maybe an option to display the log file? or to display the parser error? or bring up a terminal window?

One of these is better than just immediately exiting, especially when people start using wlmaker as a login compositor rather than running in a window.

This might apply to other plists. I didn't check.

[phkaeser]

As a mitigation to test a new root menu file:

• Run wlmaker under X11 or a Wayland compositor -- it will launch as a window.
• There, use `--
• recommending to use the --root_menu_file commandline argument to point to the new menu file.
  A bad root menu file will the process fail immediately, and permit quick iteration.

Longer-term, it indicates that wlmaker will need some form of highlighting errors to users (eg. an error dialog or notification).

Falling back to a built-in default is a good idea; but ideally should be paired with a notification about having failed to load the configured file.