[PG] adds warnings about missed migrations

[Aeolun]

The postgres migrator would silently skip migrations that were merged in from older branches if a newer migration had already been executed. If something went wrong, it also wasn't very clear what statement/migration caused the issues.

What has changed:

• The migrator will now throw specific DrizzleError errors with appropriate contextual data when something goes wrong during migration, instead of relying on the message inherent to the failure.
• A new error condition has been added when a migration will be skipped entirely while not yet existing in the migration table.

Potential further work:

• Extend this to other dialects
• Generalize it so code for the migrations isn't duplicated across dialects