## 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
