Throw exception when color can't be parsed

Before this commit, when I try to parse “rebeccapurple” ([ref](https://github.com/jonathanKingston/rework-rebeccapurple#explanation)) I get rgb(0,0,0) & don’t know if it’s really black or just an unknown value.

It will help us to ignore unknown values in [css-color-function](https://github.com/ianstormtaylor/css-color-function) & don’t return wrong value instead.