Log exception data in JSON log

When logging in JSON format, the exception data, e.g. stacktrace, which
the replication plugin logs into its own log are not logged to file.

The implementation was copied from
`com.google.gerrit.pgm.util.ErrorJsonLogEntry` in Gerrit core.

Change-Id: I690b9c21584d98dc69eabaf41fb2d4365f734ce0