throw a useful error message when the timeline is empty

[jodeleeuw]

this just got me. forgot to push anything onto the timeline and the error was cryptic at best.

[aridyckovsky]

Is the error also thrown when the timeline is undefined? Only when the timeline is defined as an array object but remains empty timeline.length == 0? Or both? It might be helpful to create verbose error messages for each, especially given the core nature of timelines.

[jodeleeuw]

Verbose errors for both seems like the right way to go.

(This is a general problem across the project that needs attention. Lots of places where we could provide more descriptive error messages.)