Minor style and simplification cleanups

This patch does various minor style and simplification cleanups, fixing things
detected by tools such as go vet, gofmt -s, and golint.

There are no functional changes, this change is purely cosmetic, but will
enable us to run those tools more regularly now that their output is clean.

internal/queue/queue.go

@@ -26,7 +26,7 @@ const (
 )
 
 var (
-	queueFullError = fmt.Errorf("Queue size too big, try again later")
+	errQueueFull = fmt.Errorf("Queue size too big, try again later")
 )
 
 // Channel used to get random IDs for items in the queue.
@@ -90,7 +90,7 @@ func (q *Queue) Len() int {
 // Put an envelope in the queue.
 func (q *Queue) Put(from string, to []string, data []byte) (string, error) {
 	if q.Len() >= maxQueueSize {
-		return "", queueFullError
+		return "", errQueueFull
 	}
 
 	item := &Item{

internal/queue/queue_test.go

@@ -117,7 +117,7 @@ func TestFullQueue(t *testing.T) {
 
 	// This one should fail due to the queue being too big.
 	id, err := q.Put("from", []string{"to"}, []byte("data"))
-	if err != queueFullError {
+	if err != errQueueFull {
 		t.Errorf("Not failed as expected: %v - %v", id, err)
 	}