Reformat webmachine errors to be lager-compatible

Lager has a built in transformation to pretty print webmachine errors,
or any error_logger event starting with "webmachine error"
(https://github.com/basho/lager/blob/master/src/error_logger_lager_h.erl#L186).
It expects two arguments.  The three argument form was crashing lager with a
badmatch error.