bugfix: fetch prefers to fetch local mirrors over remote resources (#13545)

- [x] insert at beginning of list so fetch grabs local mirrors before remote resources
- [x] update the S3FetchStrategy so that it throws a SpackError if the fetch fails.  
      Before, it was throwing URLError, which was not being caught in stage.py.
- [x] move error handling out of S3FetchStrategy and into web_util.read_from_url()
- [x] pass string instead of URLError to SpackWebError