Raise an error if preversion versions are too low

Right now the only automatic version number we have is 'last release',
but thats enough for preversioning to be invalid. When we get API and
ABI break support, this will do more useful things like telling us the
next version is not sufficiently higher.

implements: blueprint pbr-semver
sem-ver: feature
Change-Id: I2583c7409d51364ef9d84b39356d5608a95cb6f8
Merge "Raise an error if preversion versions are too low"