If you don't format `spack.yaml` correctly, `spack config edit` still fails and
you have to edit your `spack.yaml` manually.
- [x] Add some code to `_main()` to defer `ConfigFormatError` when loading the
environment, until we know what command is being run.
- [x] Make `spack config edit` use `SPACK_ENV` instead of the config scope
object to find `spack.yaml`, so it can work even if the environment is bad.
Co-authored-by: scheibelp <scheibel1@llnl.gov>
