[web-animations-1] Be consistent about describing the exceptions a method can throw

[birtles]

From @birtles on July 9, 2015 23:58

We sometimes list the exceptions thrown, e.g.
http://w3c.github.io/web-animations/#dom-animation-finish
But other times we don't, e.g.
http://w3c.github.io/web-animations/#dom-animation-reverse (this can throw for a couple of different reasons)

We should probably list the exceptions in all cases, or none of them, rather than haphazardly.

Copied from original issue: w3c/web-animations#101