Add chat messages even when the image load fails.

Some bots send images as URIs rather than base64, which can result in
them not loading properly, either due to dead links or HTTPS cert
problems. In such cases, I think its better to display the imageless
message anyway, instead of just having the client drop it entirely.
Merge pull request #150 from tec27/dead_images

Add chat messages even when the image load fails.