This commit updates `DiscourseLogstashlogger` to add the
`exception_class` and `exception_message` field to the log line when the
`progname` of the log message is `web-exception` which is Logster's
logging of exceptions during a web request.
The `exception_class` and `exception_message` fields allows consumers of
the logs to easily group logs together.
af2bd4cc50