Code generation updates

* I realised the `respond-to` media query already handles incorrect breakpoint names so i removed the check from the `generate-at-breakpoints` mixin.
* Updated the `generate-percentage-classes-at-breakpoints` mixin to handle the palm mq correctly
* `generate-percentage-classes-at-breakpoints` handles "all" as a breakpoint option
Can you add some warnings/errors? Every mixin should have em.