Bug 12965: Prevent to erase an existing item type

Bug 12965: Prevent to erase an existing item type

On creating an item type, if it already exists, it will replace the
existing one.
This patch prevent that and display a message to the interface.

Note: The fields are lost.

Test plan:
1/ Create an item type 'AAA', description 'AAA'
2/ Edit it, update the description with 'BBB'
3/ Create an item type 'AAA' with a description 'CCC' => you should get
a warning "already exists".

Works well, no errors

Signed-off-by: Amit Gupta <amit.gupta@informaticsglobal.com>
Signed-off-by: Joonas Kylmälä <j.kylmala@gmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Warning message is triggered.
Adding, editing and deleting item types still works.
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
(cherry picked from commit eb12ee1e223b662cd1e89c0a56a5d1c083ba1ddc)
Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>