function processErrors (errors, transformers) {
  const transform = (error, transformer) => transformer(error);
  const applyTransformations = (error) => transformers.reduce(transform, error);

  return errors.map(extractError).map(applyTransformations);
}
Applies all transformers to all errors and returns "annotated"
errors.

Each transformer should have the following signature WebpackError => AnnotatedError

A WebpackError has the following fields:
- message
- file
- origin
- name
- severity
- webpackError (original error)

An AnnotatedError should be an extension (Object.assign) of the WebpackError
and add whatever information is convenient for formatting.
In particular, they should have a 'priority' field.

The plugin will only display errors having maximum priority at the same time.

If they don't have a 'type' field, the will be handled by the default formatter.