In order to properly deal with exceptions on the application listening side of Log4j events from a JSON or XML source (another app in another JVM), we do not get a Throwable because the exception class may not exist in the receiving class loader. Instead, we use a ThrowableProxy to track as much information as we think will be of interest. HOWEVER, this is not available to users of the LogEvent interface. We need to make it available for other appenders which can then log the information in the proxy. The upcoming JSON and XML listeners and appender will make use of this.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1593597 13f79535-47bb-0310-9956-ffa450edef68