GraphQL: Handle missing reference object errors

'Hole in the graph' kind of errors. Reference (ID/SWHID) exists but object is missing. proposed solution is a schema change

type Arc {
           target: Snapshot | Release | Revision | Directory | Content
           type: String!  # release/revision/directory/content
           swhid: SWHID!
         }

        type Release implements MerkleNode & Node {
          id: ID!
          SWHID: SWHID!
          name: String
          message: String
          author: Person
          date: DateTime
          target: Arc
        }

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Migrated from T4518 (view on Phabricator)