Stacktrace with unsupported option in GWC Memory Blobstore

The in-memory blobstore can be enabled from the GeoServer GUI.

When using the in-process one (i.e. GUAVA based) not all the eviction policies are supported.
Problem is that we get a nasty stacktrace if we try configure one which is not supported.
We should do much better than this as we know upfront which are supported and which ones are not.

I would also update the doc for the cache.

See doc for reference:
http://docs.geoserver.org/latest/en/user/geowebcache/config.html#configuring-in-memory-caching
https://github.com/google/guava/wiki/CachesExplained#eviction