fix 'ReferenceError: error is not defined'

[caseman]

There is no variable 'error' defined, so the error case fails. I changed it to throw a new generic Error which fixes the problem.

[coveralls]

Coverage remained the same when pulling 54423a3 on caseman:patch-1 into 563efcf on markusn:master.

[markusn]

Hi!

Thanks for the PR!

Do you have any example input which triggers the throws?

[caseman]

Yeah, I triggered it by passing NaNs to closest() accidentally, e.g.:

diff.closest({R:NaN, G:NaN, B:NaN}, [{R:0, G:0, B:0}])

[markusn]

Thanks!