* op-node: Restore previous unsafe chain using backupUnsafe

* op-e2e: Enable custom error while mocking L2 RPC error

* op-e2e: Add BackupUnsafe tests

* op-node: Fix comment

* op-node: Follow convention for backup unsafe head metric

* op-e2e: Fix BackupUnsafe tests

* op-node: Tailered/Consistent log message

* op-e2e: Better coding style

* op-node: Refactor code for trying backupUnsafe reorg

* op-node: Better variable name

* op-e2e: Remove global variable

Test are run concurrently so accessing shared global object is problematic