Improve warning/error message imports about purser

[rdig]

Basically the way the react project does it is to declare the error string when it happens (no imports) and then, when building in production, just replace the warning() function with an empty one.

Combine this with an dead code eliminator like UglifyJS and you get rid of warning/error messages in production.

Code looks something like this: (simplified vs. the original implementation)

/* warning.js */

let  warning = () => {};

if (DEV) {
  warning = message => console.log(message);
}

export warning;

...

/* module.js */

import warning from './warning';

const checkIfTrue = value => {
  if (value) {
    return true;
  }
  warning('Your value: %s, is not true', value);
  return false;
};

It's kind of crude, but it works.