Migration tools, warnings and exception handling

[blythed]

If a user is on 0.5, we should be able to assist them to transition to 0.5+.

[fnikolai]

With the current approach, we do not verify the code version when connecting to the same database. This can lead to multiple conflicting or erroneous versions.

To address this, we need:

A mechanism to negotiate the schema version and deny connections if the schema is not installed.

A tool for smoothly upgrading from one version to another.

A system for managing multiple schema versions concurrently.