Update image viewer to return an actual error #210
Description
The image viewer was treating error strings from CardImage as image data and rendering them as if they were sixel output. The flow needs to be updated so that errors are shown as errors and image data is only rendered when the fetch succeeds.
◦ imageReadyMsg now includes err alongside sixelData
◦ fetchImageCmd returns imageReadyMsg{err: err} on failure and leaves sixelData empty
◦ ImageModel.Update sets m.Error on failure and only sets m.ImageData on success
◦ ImageModel.View renders a styled error when m.Error is present; otherwise renders the image data