Review web-app for inappropriate 500 status codes

[wowasa]

Apparently there are still some returned 500 status codes in the web-app, where a 404 status code would be appropriate. We have to assure that for all calls to static reports we return a 404 status code if the file is not available. Hence we check first for existence of the file and only return a 500 status code if the file is available but throws an exception during processing

[wowasa]

done