core: wrong messages in AddExistingFileStorageDomain

AddExistingFileStorageDomainCommand -> checkExistingStorageDomain:
swapped usages of ACTION_TYPE_FAILED_STORAGE_DOMAIN_ALREADY_EXIST
and ACTION_TYPE_FAILED_STORAGE_DOMAIN_NOT_EXIST (as the current
output is incorrect).

Change-Id: Idebeacf0b9332b54cfbdc62d16da89d8ce3478da
Signed-off-by: Daniel Erez <derez@redhat.com>