Include response body in non-200 OK error text.

When a GraphQL server returns a non-200 OK status code, it may be
very helpful to for caller to know what the response body was.
So, when such an error occurs, fetch the response body and
include it in the returned error text.

Fixes #29.
Updates #24.

graphql.go

@@ -5,6 +5,7 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
+	"io/ioutil"
 	"net/http"
 
 	"github.com/shurcooL/go/ctxhttp"
@@ -70,7 +71,8 @@ func (c *Client) do(ctx context.Context, op operationType, v interface{}, variab
 	}
 	defer resp.Body.Close()
 	if resp.StatusCode != http.StatusOK {
-		return fmt.Errorf("unexpected status: %v", resp.Status)
+		body, _ := ioutil.ReadAll(resp.Body)
+		return fmt.Errorf("non-200 OK status code: %v body: %q", resp.Status, body)
 	}
 	var out struct {
 		Data   *json.RawMessage
@@ -79,11 +81,13 @@ func (c *Client) do(ctx context.Context, op operationType, v interface{}, variab
 	}
 	err = json.NewDecoder(resp.Body).Decode(&out)
 	if err != nil {
+		// TODO: Consider including response body in returned error, if deemed helpful.
 		return err
 	}
 	if out.Data != nil {
 		err := jsonutil.UnmarshalGraphQL(*out.Data, v)
 		if err != nil {
+			// TODO: Consider including response body in returned error, if deemed helpful.
 			return err
 		}
 	}

graphql_test.go

@@ -101,6 +101,30 @@ func TestClient_Query_noDataWithErrorResponse(t *testing.T) {
 	}
 }
 
+func TestClient_Query_errorStatusCode(t *testing.T) {
+	mux := http.NewServeMux()
+	mux.HandleFunc("/graphql", func(w http.ResponseWriter, req *http.Request) {
+		http.Error(w, "important message", http.StatusInternalServerError)
+	})
+	client := graphql.NewClient("/graphql", &http.Client{Transport: localRoundTripper{handler: mux}})
+
+	var q struct {
+		User struct {
+			Name graphql.String
+		}
+	}
+	err := client.Query(context.Background(), &q, nil)
+	if err == nil {
+		t.Fatal("got error: nil, want: non-nil")
+	}
+	if got, want := err.Error(), `non-200 OK status code: 500 Internal Server Error body: "important message\n"`; got != want {
+		t.Errorf("got error: %v, want: %v", got, want)
+	}
+	if q.User.Name != "" {
+		t.Errorf("got non-empty q.User.Name: %v", q.User.Name)
+	}
+}
+
 // Test that an empty (but non-nil) variables map is
 // handled no differently than a nil variables map.
 func TestClient_Query_emptyVariables(t *testing.T) {