Respect log level overriding from kwargs in Exception handler

Currently the `raven.events.Exception` handler hardcodes the logging level to `logging.ERROR`. We have a use case wherein we expect certain errors to come up, but would like to log them to Sentry at a lower level, because they are expected. This actually happens if `level` is specified in kwargs for `Client.capture`, but is not propagated down to the handler. This is the easiest solution, but we could also specify another handler for warning exceptions.