Follow-up to #12226. The migrations worker is a queue worker, not an HTTP
endpoint — its errors never become HTTP responses — so they shouldn't be
drawn from the HTTP error registry (app/config/errors.php). Throw
Utopia\Migration\Exception for the source/destination setup validation
failures instead; the worker's outer-catch gate already suppresses
Migration\Exception from Sentry and records it on the migration document.
Removes the now-unused MIGRATION_SOURCE_*/MIGRATION_DESTINATION_TYPE_INVALID
constants and their errors.php entries.