Throw custom Exception when too many empty sets were iterated

[dmfs]

At present it's difficult to distinguish the IllegalArgumentExceptions thrown due to too many empty recurrence sets from others (also see #137). We could use a custom subclass that we can catch to handle those cases separately.

[dmfs]

Looks like in all but one place an IllegalArgumentException is thrown. This will be a breaking change for people who catch the IllegalStateException thrown by BySetPos.