Fix Sass compilation rebuild after error

[aduth]

🛠 Summary of changes

Fixes @18f/identity-build-sass to rebuild stylesheets as expected after saving a file that had previously triggered an error.

This relied on parsing a stack trace produced by Sass. It appears that this stack trace was updated to produce paths relative from the compiler executable.

For future consideration:

• Open an issue to sass/embedded-host-node to provide full compilation result in an error. This is related to Provide a callback for when a file is being loaded, a way to provide an always used loader or an Watch option sass/embedded-host-node#190 . A failed compilation includes loadedUrls, but there's no way to get at these since the failed result raises an error without the full result object.
• Create end-to-end test for using CLI to compile and watch files, including this scenario

📜 Testing Plan

1. yarn build-sass app/components/click_observer_component.scss --watch --verbose
2. Edit and save app/components/click_observer_component.scss to cause an error, e.g. removing a curly brace

   - diff --git a/app/components/click_observer_component.scss b/app/components/click_observer_component.scss
   index 0d9671bba4..1c5c14cb44 100644
   --- a/app/components/click_observer_component.scss
   +++ b/app/components/click_observer_component.scss
   @@ -1,3 +1,2 @@
    lg-click-observer {
      display: contents;
   -}

3. Observe error in terminal
4. Edit and save app/components/click_observer_component.scss to fix error
5. Observe in terminal that rebuild occurs as expected

   Building files [ 'app/components/click_observer_component.scss' ]
   

[aduth]

• Open an issue to sass/embedded-host-node to provide full compilation result in an error. This is related to Provide a callback for when a file is being loaded, a way to provide an always used loader or an Watch option sass/embedded-host-node#190 . A failed compilation includes loadedUrls, but there's no way to get at these since the failed result raises an error without the full result object.

See sass/sass#3830