Failover Logger

[oxinabox]

I thought the DemuxLogger was alone in its class.
but there exists at least one other.
The FailoverLogger
which is kind of like a DemuxLogger,
but instead of writing to all of its child loggers,
it tries to write to the first, then if an error is thrown,
tries the second,
and so forth.

See Go's Log15 library:

https://github.com/inconshreveable/log15/blob/67afb5ed74ec82fd7ac8f49d27c509ac6f991970/handler.go#L210-L225