Log collecting cleanup cache on local disk

[matthiasdeblock]

Check how often we receive the below error message.

Alba ticket: https://github.com/openvstorage/alba_ee/issues/276

Full stack:

2018-03-13 13:41:49 071393 +0100 - DEV-3N-199-182 - 28516/0000 - alba/proxy - 30006 - warning - adding "fragment" to cache failed; ignoring error; cleaning up blob: (Failure "None while Some was expected"); backtrace:; Raised at file "pe
rvasives.ml", line 32, characters 17-33; Called from file "src/blob_cache_fs.ml", line 36, characters 10-27; Called from file "src/blob_cache_fs.ml", line 732, characters 30-52; Called from file "rocks_common.ml", line 91, characters 8-1
2; Re-raised at file "rocks_common.ml", line 95, characters 21-30; Called from file "rocks_common.ml", line 91, characters 8-12; Re-raised at file "rocks_common.ml", line 95, characters 21-30; Called from file "src/blob_cache_fs.ml", lin
e 771, characters 50-82; Called from file "src/blob_cache_fs.ml", line 843, characters 40-73; Called from file "src/blob_cache_fs.ml", line 910, characters 6-19; Called from file "src/core/lwt.ml", line 1895, characters 23-26;