Error formating.

- replaced fmt.Errorf() with error.New(), where ever possible.
- error values are organized on namespace and an optional message.

Change-Id: I4537195c7bb40e882f80f65b6902dec3ba39e0d4
Reviewed-on: http://review.couchbase.org/38174
Reviewed-by: Manik Taneja <manik@couchbase.com>
Reviewed-by: John Liang <johliang00@gmail.com>
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Tested-by: Pratap Chakravarthy <prataprc@couchbase.com>