make resolve.sync traces more helpful

For folks who didn't notice the extraction of derby-stylus and derby-less, the resolve.sync calls throw cryptic errors. This PR catches [the resolve error](https://github.com/substack/node-resolve/blob/0.6.3/lib/sync.js#L32) and re-throws with the searched extensions.

I know the try-catch is ugly, but execution never reaches `if (!resolved)` is because of the error thrown by `resolve`.

Followup to comment on #415.