oauth2: add error type for unsuccessful token endpoint status

Allows the HTTP response and body to be extracted without parsing
the error string, but keeps backwards compatibility for users who
are currently doing so.

Fixes golang/oauth2#173

Change-Id: Id7709da827a155299b047f0bcb74aa8f91b01e96
Reviewed-on: https://go-review.googlesource.com/84156
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

oauth2_test.go

@@ -419,6 +419,32 @@ func TestFetchWithNoRefreshToken(t *testing.T) {
 	}
 }
 
+func TestTokenRetrieveError(t *testing.T) {
+	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.String() != "/token" {
+			t.Errorf("Unexpected token refresh request URL, %v is found.", r.URL)
+		}
+		w.Header().Set("Content-type", "application/json")
+		w.WriteHeader(http.StatusBadRequest)
+		w.Write([]byte(`{"error": "invalid_grant"}`))
+	}))
+	defer ts.Close()
+	conf := newConf(ts.URL)
+	_, err := conf.Exchange(context.Background(), "exchange-code")
+	if err == nil {
+		t.Fatalf("got no error, expected one")
+	}
+	_, ok := err.(*RetrieveError)
+	if !ok {
+		t.Fatalf("got %T error, expected *RetrieveError", err)
+	}
+	// Test error string for backwards compatibility
+	expected := fmt.Sprintf("oauth2: cannot fetch token: %v\nResponse: %s", "400 Bad Request", `{"error": "invalid_grant"}`)
+	if errStr := err.Error(); errStr != expected {
+		t.Fatalf("got %#v, expected %#v", errStr, expected)
+	}
+}
+
 func TestRefreshToken_RefreshTokenReplacement(t *testing.T) {
 	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		w.Header().Set("Content-Type", "application/json")