Validate upgrades before making any changes

Split the upgrade sequence into a two phase process: validate then upgrade, where validate will do its best to check that upgrades will run without a hitch, ensuring we fail fast if there are any errors. This will also mean that our users typically won't have to roll back their installation if it fails, and (if the validation can diagnose the problem sufficiently) can mean the users can correct and rerun the upgrade without needing to contact support.

This means that fully diagnosed validation errors should NOT result in the user seeing an error telling them to raise a support request. Validation errors that are surprising, however, should still come with the standard upgrade failed banner (minus any text about rolling back).