Add a fake 'bad' printer driver

pdrivers.badprinter is a printer that is always offline, for testing.
Better handling of kitchen printer problems

If the order can't be sent to the kitchen printer, it is sent to the
local receipt printer instead.  If the order can't be sent to either
printer, don't take it.

Closes #37 on github.
Only soft-lock the register if it has the focus

If the user presses "lock" when there is a popup on top of the register
display, go directly to the lock page.  Don't change the register status
to "locked" underneath the popup.