Bug 12508: adding an error message if a contract cannot be removed

If a contract cannot be removed, no errors is displayed. Now, it displays an dialog messag error and the list of contracts.

Test plan:
1) Log on with a superlibrarian permission
2) Go on the page acqui/supplier.pl (Acquisitions > Button "New vendor")
3) Record a vendor with a nonzero "name"
4) Go on the page admin/aqcontract.pl (click on the "Contracts" item in the menu)
5) Click on the button "New" > "Contract" and record a new one
6) Click on "New" > "Basket" and record a basket by selectioning the created contract
7) Click on the contract name, then click on the "Contracts" item in the left menu
8) Try to delete this contract, an error is displayed : "Contract has not been deleted. Maybe a basket linked to this contract exists."
9) Delete the basket linked to the contract
10) Delete the contract, no errors is displayed

Signed-off-by: Juhani Seppälä <jseppal@student.uef.fi>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Passes QA script an tests.
Also tested adding, editing and deleting unused contracts.

Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>
Bug 12508: Follow up - corrects capitalization

Corrects 2 strings:
- 'Delete this contract'
- 'Contract deleted'

Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>