fix(db): do not panic when undo migration SQL is missing - #4103

Bug and impact

Commit 0bd3589a changed TryRollbackMigration to call getVersionSQL(..., ignoreErrors=false). Only ~31 of ~98 migrations ship a .err.sql undo script. For any migration without one, rollback now panics instead of logging a warning and skipping.

Concrete triggers:

1. Failed migration auto-rollback — db.Migrate calls TryRollbackMigration when ApplyMigration fails. If the failing migration has no .err.sql (most do not), the Semaphore process crashes instead of returning the migration error.
2. Manual downgrade — db.Rollback iterates applied migrations and calls TryRollbackMigration for each. Rolling back past any version without undo SQL panics the process.

Root cause

getVersionSQL panics on a missing embedded file when ignoreErrors is false. The change was likely intended to surface missing undo SQL for v2.20.1, but TryRollbackMigration is invoked for all migrations during rollback — not only those with undo scripts.

Fix

• Restore ignoreErrors=true in TryRollbackMigration so missing .err.sql files are handled gracefully (warn + skip), as before 0bd3589a.
• Keep the new v2.20.1.err.sql undo script added in that commit.
• Add unit tests locking in the ignoreErrors behavior.

Validation

• go test ./db/sql -run TestGetVersionSQL -count=1 — pass