Catch DBDuplicateEntry instead of IntegrityError

'iscsi_target_create_safe' method didn't catch DB exceptions
correct.

Need to catch both db_exc.DBError and db_exc.DBDuplicateEntry
due to current oslo.db limitations: it doesn't handle correct
DBDuplicateEntry with different SQLite versions

Related-Bug: #1187367

Change-Id: Ie412ecabd3cbcac301340c82b4275adb7a7206c3
Merge "Catch DBDuplicateEntry instead of IntegrityError"