Use file cache on load error

[elliottinvent]

At the moment we cached files we've previously loaded. If the file is unavailbale due to server downtime or another reason, we should use the cached file for as long as possible upto maybe 7 days. At the moment, when the one hour cache expires and the file is unavailable the load fails.

[twalmsley]

Will be fixed in v0.3.12 - the code is updated but needs a proper test in the playground before the issue can be closed.