Video Player: minor fixes

In this PR are fixed minor issues mentioned in PR #637 .

1) Is it necessary to assert False if mode isn't present? Seems like we could just assume that whoever's writing the feature will use the right mode. 
2) It's usually good to end steps (especially short ones) with a $ regex anchor -- we've had bugs that weren't caught because two definitions happened to match a step, and the wrong test was chosen by Lettuce. Making these definitions as specific as possible can avoid this.
3) Minor nitpick, but this and line 24 should probably be one for nicer formatting.

@peter-fogg review, please.