Bug 9860 [Alternate] Incorrect box heading 'Cannot check in' when a late document...

Bug 9860 [Alternate] Incorrect box heading 'Cannot check in' when a late document is returned

The fix for Bug 9397 moved the "cannot check in" heading from within the
one conditional where it should have been displayed to the top of all
check-in errors/messages. "Cannot check in" is still only valid if the
item is withdrawn and BlockReturnOfWithdrawnItems is ON.

In order to accomplish the goal of Bug 9397 while fixing Bug 9860 this
patch puts "cannot check in" back into the correct conditional and adds
a generic heading to check in messages for accessibility purposes.

To test, check in items which will trigger a variety of errors
including:

- item must be returned to home library
- item is lost
- item is withdrawn (wtih BlockReturnOfWithdrawnItems on and off)
- item is overdue

...as well as items which should check in normally. Messages should
display correctly.

Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>