Throw exception color can't be parsed

Before this commit, when I try to parse “rebeccapurple”
([ref](https://github.com/jonathanKingston/rework-rebeccapurple#explanat
ion)) 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-functio
n) & don’t return wrong value instead.
Merge branch 'exception'

Conflicts:
	color.js
	test/basic.js
version to v0.7.1