Adding better error message for missing mixin

[justin808]

Can I add an informative message to:
https://github.com/facebook/react/blob/master/src/class/ReactClass.js#L430

in case that we try to mixin either nil, null, or {}

(basically, I'll submit a patch if maintainers think this is a good idea)

[sophiebits]

See #1302 and #1305 – we'd like to do this but last time it wasn't possible due to some mocking problems.