Bug 16097: Making messages for subscription fields more user friendly

Bug 16097: Making messages for subscription fields more user friendly

To test:
1) Go to Serials -> Add Subscription fields
2) Create a field, confirm message.
3) Attempt to create a new field that has the same name as an existing
   field, confirm message.
4) Create another field with a different name. Attempt to edit this
   field to have the same name as another field. Confirm message
5) Edit field (change name to something different)
6) Delete field, confirm message.
6) Turn off permissions to delete subcriptions ONLY. Attempt to delete a
   field, confirm message.

Sponsored-by: Catalyst IT
Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Brendan Gallagher brendan@bywatersolutions.com