Return HTTP 413 when server rejects document due to size

When a document exceeds the server size limit (20MB), SG should return a 413 instead of 500.  This handling was already in place for gocouchbase/gomemcached in ErrorAsHTTPStatus - it needs to get added there for gocb.

In the case where the xattr exceeds 1MB, it still makes sense to return 500, as that's more of an internal error (i.e. the request payload is not necessarily too big).