Refactor: Raise instead of return nil

If not checksummable.

No behavior change, because exceptions are caught and tracked as
failures. This is more explicit than the current logic where the
transition to `verification_succeeded` raises an error due to the `nil`
checksum.

ee/lib/gitlab/geo/replicable_model.rb

@@ -52,7 +52,7 @@ module Gitlab
       #
       # @return [String] SHA256 hash of the carrierwave file
       def calculate_checksum
-        return unless checksummable?
+        raise 'File is not checksummable' unless checksummable?
 
         self.class.hexdigest(replicator.carrierwave_uploader.path)
       end