Return clearer errors when __typename is missing (#68)

## Summary:
In practice, at Khan at least, this is easy to mess up when writing
mocks, because you write the mock by looking at the query, and the query
doesn't say it's asking for `__typename` (because genqlient
automatically adds that).  A sufficiently-smart mocking library might be
able to fix that, or detect it at least, but in any case, we can give a
clearer error.

I also removed an unrelated TODO that was done.

Issue: https://khanacademy.slack.com/archives/C01120CNCS0/p1630019788014000

## Test plan:
make check


Author: benjaminjkraft

Reviewers: dnerdy, aberkan, csilvers, MiguelCastillo

Required Reviewers: 

Approved by: dnerdy

Checks: ✅ Test (1.17), ✅ Test (1.16), ✅ Test (1.15), ✅ Test (1.14), ✅ Test (1.13), ✅ Lint, ✅ Test (1.17), ✅ Test (1.16), ✅ Test (1.15), ✅ Test (1.14), ✅ Test (1.13), ✅ Lint

Pull request URL: https://github.com/Khan/genqlient/pull/68

generate/testdata/snapshots/TestGenerate-UnionNoFragments.graphql-UnionNoFragments.graphql.go

@@ -61,6 +61,9 @@ func __unmarshalUnionNoFragmentsQueryRandomLeafLeafContent(v *UnionNoFragmentsQu
 	case "Video":
 		*v = new(UnionNoFragmentsQueryRandomLeafVideo)
 		return json.Unmarshal(m, *v)
+	case "":
+		return fmt.Errorf(
+			"Response was missing LeafContent.__typename")
 	default:
 		return fmt.Errorf(
 			`Unexpected concrete type for UnionNoFragmentsQueryRandomLeafLeafContent: "%v"`, tn.TypeName)
@@ -98,7 +101,8 @@ func (v *UnionNoFragmentsQueryResponse) UnmarshalJSON(b []byte) error {
 		err = __unmarshalUnionNoFragmentsQueryRandomLeafLeafContent(
 			target, raw)
 		if err != nil {
-			return err
+			return fmt.Errorf(
+				"Unable to unmarshal UnionNoFragmentsQueryResponse.RandomLeaf: %w", err)
 		}
 	}
 	return nil