Improve Style Conflict Error Message

[amiller-gh]

See: https://github.com/css-blocks/css-blocks/pull/62#discussion_r167109504

tl;dr, move template usage data to the error message:

The following property conflicts must be resolved for these co-located Styles:

  template:
    div.class always uses block-a.root (templates/my-template.hbs:10:32)
      and conditionally uses block-b.root (templates/my-template.hbs:10:39)

  color:
    block-a.root (blocks/foo.block.css:3:36)
    block-b.root (blocks/b.block.css:1:30)

  background-color:
    block-a.root (blocks/foo.block.css:3:48)
    block-b.root (blocks/b.block.css:1:43)

[milandimun]

@amiller-gh is it still open? I could work on this.