_loader.py: Some fixes in how we load sources for junctions

  o Ensure that we call Source._load_ref(), and consider project.refs this way

  o Ensure that we report a warning in the case that project.refs is in use
    and the loaded junction source has a redundant ref which is going to
    be ignored

  o Report more distinct machine readable errors for failures to load
    junction element sources

  o Handle Consistency.INCONSISTENT and Consistency.RESOLVED separately:

    - The user should be told something different depending on whether
      they need to fetch or whether they need to track.

    - It is never possible to automatically fetch the source in the
      case that the source has no ref to begin with.

This also adjusts the test/loader/junctions.py test to expect the new error