Use check_call() in more places, since failing silently is bad.
Fix raise_from definition and usage.
Revert "Don't ignore _*.scss here at sass.  Manifest is probably better."

This reverts commit 2c715ad1f1aab147c69e970d0c431d9aa0921d99.
Make sure we use posixpath handling for input name.
If we get file not found from a target we're installing, skip it.