migration_manager: Do not swallow exception in migration_task::run_may_throw
migration_manager: Do not swallow exception in migration_task::run_may_throw

The user migration_manager::submit_migration_task needs to know if
migration_task::run_may_throw is successful or not.

Do not swallow exception.

Fixes #4791

scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla