Add message argument to ImagingError_ModeError call

Author: radarhere

src/_imaging.c

@@ -2073,7 +2073,7 @@ im_setalpha(ImagingObject *self, PyObject *args) {
     /* attempt to modify the mode of an image in place */
     Imaging im = self->image;
     if (im->mode != IMAGING_MODE_RGB && im->mode != IMAGING_MODE_RGBX) {
-        return ImagingError_ModeError();
+        return ImagingError_ModeError(NULL);
     }
     im->mode = IMAGING_MODE_RGBA;
     im->bands = 4;