[api fix] Use `flatiron.plugins.exceptions`
[api fix] Use `flatiron.plugins.exceptions`

`exceptions` plugin is not loaded by default since
flatiron/broadway@bf5a77e455cdcc240cff271d16af89d2b4b85519.

frankrousseau/cozy-controller-old,nodejitsu/haibu,frankrousseau/cozy-controller-old,nodejitsu/haibu