Avoid putting incorrect extensions on error snapshot files (#92)

## Summary:
They were accidentally ending up as `.go` or `.graphql`, but are
actually just text.

Issue: https://github.com/Khan/genqlient/issues/90

## Test plan:
make check


Author: benjaminjkraft

Reviewers: StevenACoffman, dnerdy

Required Reviewers: 

Approved By: StevenACoffman

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

Pull Request URL: https://github.com/Khan/genqlient/pull/92

generate/testdata/snapshots/TestGenerateErrors-MissingTypeName.go

@@ -1 +0,0 @@
-testdata/errors/MissingTypeName.schema.graphql:2: union/interface type I must request __typename

generate/testdata/snapshots/TestGenerateErrors-MissingTypeName.graphql

@@ -1 +0,0 @@
-testdata/errors/MissingTypeName.schema.graphql:2: union/interface type I must request __typename