Add mailet processing tests for `NoSuchMethodError`

The error could be thrown upon an extension mailet relying on old James's API.

Today James mailet pipeline is not resilient on that error, we should be more resilient on that.

Add tests for 3 cases of `onMailetException`:
- default: should trigger error processor
- propagate: propagate error to `JamesMailSpooler`
- ignore: error should be ignored
2 files changed