Closes #279: also save and show an error message

If a image fails to load, an error message is also saved in the HashMap
FAILED_IMAGES. This map was previously a set. If the user clicks on a
Stadtbild in the aggregation renderer, and the image has an entry, then
its message is shown in beanmill.
Merge branch 'feature/279-show-error-message-stadtbild' into dev