MAINT-1481: minor bug fix plus incomplete UI tweaks.

Test for existence of target name using -e rather than -f. (-d would work too,
but in this case we must respond to any name collision, whether file or
directory.)
Instead of terminating on failure, make sudo_mv return rc of the [sudo] mv
command to its caller. If the attempt to move new install to actual viewer
directory fails, restore previous viewer before failing.
When redirecting the script's stderr to updater.log, first save existing
stderr to another file descriptor, and restore it when we launch viewer.
Otherwise updater.log ends up collecting the viewer's duplicate stderr log
output!
The construct 'exec ... | program' doesn't work. In fact it causes any other
redirections on that command to fail too. Remove it -- real fix pending.