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/generated.go

@@ -266,6 +266,17 @@ func (v *UserFields) UnmarshalJSON(b []byte) error {
 	return nil
 }
 
+// failingQueryMeUser includes the requested fields of the GraphQL type User.
+type failingQueryMeUser struct {
+	Id string `json:"id"`
+}
+
+// failingQueryResponse is returned by failingQuery on success.
+type failingQueryResponse struct {
+	Fail bool               `json:"fail"`
+	Me   failingQueryMeUser `json:"me"`
+}
+
 // queryWithFragmentsBeingsAnimal includes the requested fields of the GraphQL type Animal.
 type queryWithFragmentsBeingsAnimal struct {
 	Typename string                                       `json:"__typename"`
@@ -1074,6 +1085,30 @@ query simpleQuery {
 	return &retval, err
 }
 
+func failingQuery(
+	ctx context.Context,
+	client graphql.Client,
+) (*failingQueryResponse, error) {
+	var err error
+
+	var retval failingQueryResponse
+	err = client.MakeRequest(
+		ctx,
+		"failingQuery",
+		`
+query failingQuery {
+	fail
+	me {
+		id
+	}
+}
+`,
+		&retval,
+		nil,
+	)
+	return &retval, err
+}
+
 func queryWithVariables(
 	ctx context.Context,
 	client graphql.Client,