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/unmarshal_helper.go.tmpl

@@ -17,10 +17,15 @@ func __unmarshal{{.GoName}}(v *{{.GoName}}, m {{ref "encoding/json.RawMessage"}}
     switch tn.TypeName {
     {{range .Implementations -}}
     case "{{.GraphQLName}}":
-        {{/* TODO: handle repeated fields! */ -}}
         *v = new({{.GoName}})
         return {{ref "encoding/json.Unmarshal"}}(m, *v)
     {{end -}}
+    case "":
+        {{/* Likely if we're making a request to a mock server and the author
+             of the mock didn't know to add __typename, so give a special
+             error. */ -}}
+        return {{ref "fmt.Errorf"}}(
+            "Response was missing {{.GraphQLName}}.__typename")
     default:
         return {{ref "fmt.Errorf"}}(
             `Unexpected concrete type for {{.GoName}}: "%v"`, tn.TypeName)