Actually skip unfetchable courses

[nmaggioni]

Courses that encountered errors while fetching their recordings lists were still included in the ones to process but contained undefined data.

This PR is limited to explicitedly marking them as null so they can later be ignored properly; the error handler was discarded as I supposed it was just a re-throwing placeholder.
A slightly better solution could be to have proper metadata initialized before fetching the details and verified afterwards, but that'd be a more involved approach that would arguably yield significant benefits.

Also moved about and updated calls for #1 to maintain compatibility even if currently disabled. Said functionality could perhaps be put under a config variable for ease of use on supported platforms at users' discretion.

[beryxz]

Nice catch!
Instead of returning null I'll see about refactoring that part to fix the error and be more verbose to the user about what went wrong.

Also moved about and updated calls for #1 to maintain compatibility even if currently disabled. Said functionality could perhaps be put under a config variable for ease of use on supported platforms at users' discretion.

As of know I don't think it is worth it just yet to re-open that functionality. For readability sake, I'll remove the comments but keep the API for future usage.