fix(images): ensure decode from URL emits error on HTTP error code

We should emit an error when the request to an image to be fetched
returns and HTTP error code. Otherwise, we'll run into other higher
level errors down the line, which are misleading

Example: I kept seeing "image content type not supported" errors,
although the content type *is* supported. The actual problem was that
the decode function operates on non existing image bytes.

images/decode.go

@@ -48,6 +48,10 @@ func DecodeFromURL(path string) (image.Image, error) {
 		}
 	}()
 
+	if res.StatusCode >= 400 {
+		return nil, errors.New(http.StatusText(res.StatusCode))
+	}
+
 	bodyBytes, err := ioutil.ReadAll(res.Body)
 	if err != nil {
 		return nil, err

images/decode_test.go

@@ -150,6 +150,14 @@ func TestDecodeFromURL(t *testing.T) {
 	}
 }
 
+func TestDecodeFromURL_WithErrors(t *testing.T) {
+	s := httptest.NewServer(http.FileServer(http.Dir(path)))
+	defer s.Close()
+
+	_, err := DecodeFromURL("https://doesnt-exist.com")
+	require.Error(t, err)
+}
+
 func TestGetType(t *testing.T) {
 	cs := []struct {
 		Buf   []byte