With this commit Spack fails with an error and an informative message,
if the local store is at an outdated version. This usually happens when
people upgrade their Spack version, but retain the same configuration as
before.
Since updating the DB without an explicit consent of the user means making
the local store incompatible with previous versions of Spack, here we opt
to error out and give the user the choice to either reindex, or update the
configuration.
ac72170e91