Define situations that should throw errors

[trajanmcgill]

Ease of use would be enhanced if clear mistakes are caught and descriptive errors thrown, rather than leaving a developer with buggy code wondering whether Concert.js is not working, or his/her own code is not working, and why. For instance, trying to add animation of a target feature that overlaps with another animation segment transforming the same target feature will never work. (Although perhaps some future capacity for modifying existing animation sequences could imaginably be added.)

[trajanmcgill]

Another case is trying to add keyframes where the number of times specified is not the same as the number of values specified.