Add support for logging Throwable as warning

[erdeszt]

I would like to log a message as a warning and include an exception in the log. So similar to the:

def error(line: => String, cause: Cause[Any])

I'd like to have a:

def warn(line: => String, cause: Cause[Any]) 

That would log at the Warn level but it would include the error trace similarly to error.

If this is already possible and I just missed it somehow I'm sorry, otherwise if there's no objection I'd be happy to work on a PR for this.

[pshemass]

Go ahead and open PR for this.