Use `console.warn` to log warnings instead of `console.error`

[baibhavx]

Do you want to request a feature or report a bug?

• Feature(Improvement)

What is the current behavior?

• We are using console.error instead of console.warn to log warnings

What is the expected behavior?

• We should use console.warn to log warnings unless console.error is being used for a good reason.

• There is a brief discussion/suggestion to use console.warn to log warnings in this thread: Suggestion: Warn when bindActionCreators encounters non-function property #2279

• Incase we decide to go with this, here is the PR with proposed changes: Use console.warn to log warnings instead of console.error #3030

[timdorr]

This is inherited from fbjs's warning, which was previously used. It's explained here: facebook/fbjs#94 (comment)