:bug: 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
:bug: Fix invalid lighten/darken operation for less/sass

Stylus being the only one that use relative percentage rather than an
absolute one, and as it’s usage is largely less widespread that Sass or
Less, the default behaviour of these two operations is no longer based
on its implementation.

Closes abe33/atom-color-highlight#26
Prepare 1.11.2 release