Http error code

This change means that whenever a `ServiceException` occurs, an HTTP status code of 500 will be returned to the client, instead of always 200 (which should play better with our squid caching).

Note the patch at http://jira.codehaus.org/browse/GEOS-4255 was not used for the following reasons:
* it's against an older version of geoserver than we are using (2.1.x)
* it touches many files, which would give it a large maintenance overhead if we were to maintain it ourselves.

So, this patch is much simpler, with the downside that 500 is *always* return for errors (instead of the appropriate 400, 500 code, as in the patch) - but - this *should* be good enough for us re squid.

Tested with the following URLs (connecting to a VM provisioned with 3-nec-mel):

WMS:
curl -I "http://10.11.12.14:8081/geoserver/aodn/wms?service=WMS&version=1.1.0&request=GetMap&layers=aodn:coastal_watch_locations&styles=&ox=138.468,-38.542,153.627,-19.148&width=400&height=51&srs=EPSG:4326&format=application/openlayers"

WFS:
curl -I "http://geoserver-rc.aodn.org.au/geoserver/imos/ows?service=WFS&version=1.0.0&request=GetFeature&typeName=imos:csiro_harvest_phyto_dsdfata&maxFeatures=50&outputFormat=csv"

I also tested hacking the JNDI entry to a bad URL and getting a 500 back at the client.