If Migrate3 has already applied its schema changes but failed to
bump user_version (e.g. process crashed between ALTER and
SetMigrationVersion), the next run currently hits a duplicate
column error because ALTER TABLE ADD COLUMN is not idempotent in
SQLite.
Detect the recovery case by checking for the presence of the v3
target columns and the absence of the dropped Code column, and
just record the migration version when found.
JonKazama-Hellion
94064bf9e4