Make sure errors are always displayed when watching for changes

[thecrypticace]

Fixes #9798

My testing also revealed an unfortunate DX problem due to the way PostCSS works:

1. Runners are promise based if any plugin is async
2. If any runners error then the promise rejects with an error
3. The error does NOT have access to the partial result that would've been built up
4. Therefore we can't get even a partial list of dependencies to watch for more changes from an initial build

As a result of this, if there is an error produced in a file that is @import-ed we won't pick it up as something that can be watched for changes until the build succeeds at least once. The workaround is to fix the error and re-save the input CSS file. I've added a TODO to find a way to work around this.