Improve Style Conflict Error Message

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)
```

Start with the linked pull-request discussion, then locate the existing style-conflict error construction and any coverage for its output. Trace how template usage data reaches the error, and consider the work complete when the message includes the template and property details shown in the issue example.

Written by the indexing model from the issue text.