MAINT-1481: Clean up update_install UI, including error output.

Capture actual error output from mkdir and mv; display it to user.
Introduce mysudo function used by sudo_mv function for graphical sudo command.
Since update_install actually only displays a single status message, just use
zenity --info instead of a zenity progress box: need not update its message.
Borrow semantics for clear_message and status functions from xmenity script.
Introduce errorbox function so we only have to make zenity/xmessage test once.
Move cleanup, onexit to top so we can use for clear_message.