scheduler: improve handling of offsets

[cebtenzzre]

These changes are inspired by #2417 to generally improve the code quality in this area.

This attempts to fix two potential issues (which I haven't made an effort to reproduce):

• passing an offset greater than the interval might cause the scheduler to do weird things later - now it throws an error immediately
• if random_start is use with a sun event, but the base offset is zero, it might schedule the event to occur again the same day - now it explicitly schedules based on the next sun event in either case

Fixes #2384

[cebtenzzre]

I just added commit 6bef390 which should fix #2384.