web-app configuration error handling

I suggest for DocumentStore.setOutDir checking if the out directory exists and throwing an error if not.

In (the very likely) case that someone doesn't follow the INSTALL file suggestion to use '/srv/onionoo' as working directory, the missing folder is reported immediatly.

And, maybe add some hint about changing 'etc/web.xml.template' to the top of INSTALL? Like "If you choose a different working directory make sure its path is reflected in the following files: ..."