change "unable to bind..." warning to a log

because it doesn't really seem to be an error, things work fine even when this is triggered
at least in updatemenus.json

Author: alexcjohnson

src/plots/command.js

@@ -114,7 +114,7 @@ exports.manageCommandObserver = function(gd, container, commandList, onchange) {
     } else {
         // TODO: It'd be really neat to actually give a *reason* for this, but at least a warning
         // is a start
-        Lib.warn('Unable to automatically bind plot updates to API command');
+        Lib.log('Unable to automatically bind plot updates to API command');
 
         ret.lookupTable = {};
         ret.remove = function() {};