Fix truncated error messages

Errors used to get truncated like this:
```
22:27:48.033 [error] Error in process <0.244.0> with exit value: {#{'__exception__'=>true,'__struct__'=>'Elixir.KeyError',key=>conn,term=>#{'__struct__'=>'Elixir.Reagent.Connection',id=>#Ref<0.0.0.1299>,listener=>#{'__struct__'=>'Elixir.Reagent.Listener',acceptors=>#'__struct__'=>'Elixir.HashSet',root=>{[],[],[],[],[],[],[],[]},...
```
This seems to solve the problem by using Task, as suggested by `fishcakez_` and `ericmj` on `#elixir-lang`.