Attempting to fetch a document from a corrupted couchstore file silently fails with ENOENT

If a couchstore file becomes corrupted such that a document cannot be fetched from disk (returning CHECKSUM_FAIL or any other non-zero status), then instead of logging and returning a error to the user (TMPFAIL), KV-Engine incorrectly returns ENOENT - i.e. document doesn't exist.

Steps to Reproduce

1. Create a document with some known value:

cbc create -U couchbase://localhost:12000/default -u Administrator -P asdasd djr -V abcdefgh

2. Lookup the vBucket ID of the key (390 for this key):

cbc hash -U couchbase://localhost:12000/default -u Administrator -P asdasd djr

djr: [vBucket=390, Index=0] Server: localhost:12000, CouchAPI: http://localhost:9500/default

3. Eject that value from memory (so future accesses require background fetch) using the attached script:

cb_evict_key.py Administrator asdasd default 390 djr

4. Manually corrupt the appropriate couchstore file (390.couch.1), by replacing the value with garbage (this will cause CRC32 to fail):

perl -pi -e 's/abcdefgh/xxxxxxxx/g' ns_server/data/n_0/data/default/390.couch.1

5. Attempt to read the document:

cbc cat -U couchbase://localhost:12000/default -u Administrator -P asdasd djr 

Expected Behaviour: TMPFAIL should be returned to the client (asking it to retry):
djr Temporary failure received from server. Try again later (0xb) and the failure should be logged in memcached.log: Failed background fetch for vb:390, seqno:9

Actual Behaviour: ENOENT is returned, and no message logged.