'Warning: ' messages should use console.warn()

[danbernardi]

Currently they use console.error(), which overestimates their severity and causes devs to react more strongly than the message might imply.

Example:

warning.js:45 
Warning: Failed propType: Invalid prop `things` of type `array` supplied to `Item`, expected `object`. Check the render method of `Connect(Item)`.

[zpao]

Thanks for the feedback but this is intentional. console.error provides good stack traces and our treatment of warnings is that they should be fixed before they are committed and shipped. With maybe 1 exception, I think they should all be fixable.

One of the things we're working towards is a better way to manage these so that you can silence specific warnings or types of warnings without having to fork React. You can track that in the umbrella issue - #5306