Handle errors due to unavailable CSS frameworks

[hernanmd]

This PR handle errors due to unavailable CSS frameworks by skipping the download and reporting the error to the Transcript (issue #693)

(Note: Inadvertently, in Pharo 12, all Symbol specifying category and class names were replaced by String. I don't know how to prevent this or send it in another PR since Iceberg doesn't list the changes if there are no changes :) If anyone knows, please let me know).

A second PR will prevent the listing of the CSS framework in the UI.