18842: Turn disk write errors into KeepCacheError

18842: Turn disk write errors into KeepCacheError

* DiskCacheSlot properly keeps track of the file handle now
* Failure to write disk block to cache will attempt to shrink cache
and try again, if it still fails, turns into KeepCacheError
* init_cache writes a test block to check for working cache

Arvados-DCO-1.1-Signed-off-by: Peter Amstutz <peter.amstutz@curii.com>