The way `error_logger` messages were forwarded to `logger` raised their level
for undetermined reasons (e.g. an info message became a notice.)

In order to fix this, invoke `logger` functions directly when they're available,
*unless* `lager` is also present and has sabotaged the default `logger` handler,
which would condemn our own `logger` messages into oblivion; in such cases, keep
using `error_logger` as `lager` will display those messages properly in any case.

Some information on the conflict between `logger` and `lager`:
* erlang-lager/lager#492
* erlang-lager/lager#488