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.go.tmpl

@@ -85,7 +85,8 @@ func (v *{{.GoName}}) UnmarshalJSON(b []byte) error {
         err = __unmarshal{{$field.GoType.Unwrap.Reference}}(
             {{if $field.GoType.IsPointer}}*{{end}}target, raw)
         if err != nil {
-            return err
+            return fmt.Errorf(
+                "Unable to unmarshal {{$.GoName}}.{{$field.GoName}}: %w", err)
         }
         {{range $i := intRange $field.GoType.SliceDepth -}}
         }