logs: removed all log.Errors unhelpful to users

Let's save log.Error for things the user can take action on.
Moved all our diagnostics to log.Debug. We can ideally reduce them
even further.

merkledag.go

@@ -136,7 +136,7 @@ func FetchGraph(ctx context.Context, root *Node, serv DAGService) chan struct{}
 
 			nd, err := lnk.GetNode(serv)
 			if err != nil {
-				log.Error(err)
+				log.Debug(err)
 				return
 			}
 
@@ -199,7 +199,7 @@ func (ds *dagService) GetNodes(ctx context.Context, keys []u.Key) []NodeGetter {
 				nd, err := Decoded(blk.Data)
 				if err != nil {
 					// NB: can happen with improperly formatted input data
-					log.Error("Got back bad block!")
+					log.Debug("Got back bad block!")
 					return
 				}
 				is := FindLinks(keys, blk.Key(), 0)