Throw a meaningful error when there's a syntax mistake in composes

Currently, `composes: a-block[type=butcher]` (value is not in quotes) results in an error that's not meaningful, like the following.
```Build Error (broccoli-css-blocks-analyze)

[css-blocks] BlockSyntaxError: No Block named "b-block" found in scope. (xxx/card.block.scss:18:3)```

We should throw an error that suggest a quoting the value instead.

Start by reproducing the unquoted `composes: a-block[type=butcher]` example and trace how the syntax is parsed and reported. The issue is done when this mistake produces a meaningful error suggesting that the value be quoted, rather than the misleading missing-block error shown in the report.

Written by the indexing model from the issue text.