Fix reflection warnings

When run with *warn-on-reflection* true, I get the following warnings:
> Reflection warning, onelog/core.clj:221:45 - reference to field getStackTrace can't be resolved.
> Reflection warning, onelog/core.clj:227:18 - reference to field getCause can't be resolved.
> Reflection warning, onelog/core.clj:238:35 - reference to field getMessage can't be resolved.