Feature request: Custom error messages and fallback image

[philipptrenz]

Hey there, yesterday (by mistake) I upgraded to v2 and am pretty impressed of the new UI and features. Awesome work!

What I already missed in v0 is the possibility to individualize error messages. A custom error text, at best with automatic localization, would be an excellent start. As an extension it would be great if instead of the black frame the last snapshot or a fallback image could be shown.