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.

fuse/readonly/readonly_unix.go

@@ -110,7 +110,7 @@ func (s *Node) Attr() fuse.Attr {
 		}
 
 	default:
-		log.Error("Invalid data type.")
+		log.Debug("Invalid data type.")
 		return fuse.Attr{}
 	}
 }