GOCBC-213: Replaced ErrInternalError use with ErrCliInternalError.

Motivation
----------
In various places throughout the Go SDK, the kv-oriented ErrInternalError
has been used to signal internal errors.  This causes confusion when
debugging as it is not possible to immediately understand where the source
of any particular internal error comes from.

Changes
-------
Imported new ErrCliInternalError error from gocbcore and updated all uses
of ErrInternalError to utilize the new client-oriented error.

Change-Id: I534891bbdd35b41a0d7b843b7e6b35270ff548d1
Reviewed-on: http://review.couchbase.org/80665
Reviewed-by: Michael Nitschinger <michael@nitschinger.at>
Tested-by: Brett Lawson <brett19@gmail.com>

cluster_n1qlquery.go

@@ -368,7 +368,7 @@ func (c *Cluster) doN1qlQuery(b *Bucket, q *N1qlQuery, params interface{}) (Quer
 
 	stmtStr, isStr := q.options["statement"].(string)
 	if !isStr {
-		return nil, ErrInternalError
+		return nil, ErrCliInternalError
 	}
 
 	c.clusterLock.RLock()

error.go

@@ -91,6 +91,8 @@ var (
 	ErrNetwork = gocbcore.ErrNetwork
 	// ErrTimeout occurs when an operation times out.
 	ErrTimeout = gocbcore.ErrTimeout
+	// ErrCliInternalError indicates an internal error occurred within the client.
+	ErrCliInternalError = gocbcore.ErrCliInternalError
 
 	// ErrStreamClosed occurs when an error is related to a stream closing.
 	ErrStreamClosed = gocbcore.ErrStreamClosed