Add tests for behavior on error (#83)

## Summary:
We guarantee that we never return a nil response, so you can safely do
```
resp, err := myQuery(...)
return resp.Field.SubField, err
```
And furthermore, if the error was a GraphQL error, `resp` may even be
nonzero; other, non-failing fields may be set.  (This depends on the
server, of course.) But we weren't testing either of those.  Now we do.

## Test plan:
make check


Author: benjaminjkraft

Reviewers: jvoll, aberkan, dnerdy, MiguelCastillo, mahtabsabet

Required Reviewers: 

Approved By: jvoll

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/83

internal/integration/server/server.go

@@ -2,6 +2,7 @@ package server
 
 import (
 	"context"
+	"fmt"
 	"net/http/httptest"
 
 	"github.com/99designs/gqlgen/graphql/handler"
@@ -82,6 +83,11 @@ func (r *queryResolver) LotteryWinner(ctx context.Context, number int) (Lucky, e
 	return nil, nil
 }
 
+func (r *queryResolver) Fail(ctx context.Context) (*bool, error) {
+	f := true
+	return &f, fmt.Errorf("oh no")
+}
+
 func RunServer() *httptest.Server {
 	gqlgenServer := handler.New(NewExecutableSchema(Config{Resolvers: &resolver{}}))
 	gqlgenServer.AddTransport(transport.POST{})