🐛 Fix css color function raising exception when invalid

As the other expressions and operations the css color function handler
now set the `isInvalid` property to true if the `convert` method raise
an exception.

Closes abe33/atom-color-highlight#27

src/color-operations.coffee

@@ -232,8 +232,11 @@ Color.addExpression 'input', "invert#{ps}(#{notQuote})#{pe}", (color, expression
 
 # color(green tint(50%))
 Color.addExpression 'css_color_function', "color#{ps}(#{notQuote})#{pe}", (color, expression) ->
- rgba = cssColor.convert(expression)
- color.rgba = new Color(rgba).rgba
+ try
+ rgba = cssColor.convert(expression)
+ color.rgba = new Color(rgba).rgba
+ catch e
+ color.isInvalid = true
 
 parseParam = (param, fileVariables={}, block) ->
  [block, fileVariables] = [fileVariables, {}] if typeof fileVariables is 'function'