core: add constant instead of harcoded msg name

Add missing message code to VdcBllMessages for validation of storage
connection format - instead of putting a hardcoded message name.
In addition, align message code name in backend appErrors.properties to the frontend
appErrors.properties to use underscore and not dots.

Change-Id: I6b8c098f0cd8330b21bc7e5c47ebb0d5e8365354
Signed-off-by: Alissa Bonas <abonas@redhat.com>