Previously, code would only look up the default sink's
loglevel, not the referenced sink name's loglevel.

In other words, if lager_event's loglevel was set to NONE,
it squelch all events from all sinks, and conversely if
a sink's loglevel was set to NONE, it would be silently
ignored.

If a sink is referenced in a call, but is not configured
(by being a running process), then emit an
{error, {sink_not_configured, Sink}} tuple.

Throw {error, bad_config} if we find one during
handler startup.

Add/update file copyright headers where deterministically appropriate.

Note:
Many updates to rebar.config files have been scripted to ensure
consistency and likely have re-ordered configuration settings and options.
While the file may look significantly different, the content other than
dependency updates (and, in some cases, more stringent compilation options)
should be the same from rebar's perspective.