[Feature] Keep downloading mods on error

[NathanPeake]

Problem

Is your feature request related to a problem? Please describe. when one mod fails to download and forces you to redownload all the mods and reselect mods

Suggestions

Describe the solution you'd like Don't stop on error, finish and then retry errored mods

[HebaruSan]

CKAN doesn't redownload mods (each one goes into the cache as soon as it finishes), but we can look into keeping selections when there's a problem.