What happens if old AddGene files disappear of if files change

[manulera]

Related to #182.

Not sure how long the urls of AddGene are kept around, so there's a chance that they will be removed and the source will no longer be possible to be re-run. Also, maybe the sequence / annotation may change. Some way of warning the user that that's the case could be good. May need something in the frontend as well.

[manulera]

So it happened. The file was accessible for a few days at https://media.addgene.org/snapgene-media/v1.7.9-0-g88a3305/sequences/110162/c1c98803-c8ba-44a6-95b8-d6a94097e36f/addgene-plasmid-65109-sequence-110162.gbk but has now been removed from AddGene (can no longer be accessed).

Unclear what should happen in this case. Perhaps return an updated source with the new url, but a warning would be appropriate maybe?