Bug 15403: Tidy up error messages in Staff lists

Bug 15403: Tidy up error messages in Staff lists

To test:
1) Create a list on the intranet, observe the message
2) Create another list with the same name, observe the message
3) Delete a list, observe the message
4) Check the patch itself for typos

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Brendan A Gallagher <brendan@bywatersolutions.com>
(cherry picked from commit f6c1b30d6f23b87bc082c55fb7fb3ecb6dd3da78)
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>