[MODEL] Added an option to the import method to return an array of error messages

Instead of returning just the number of errors, return all error messages from Elasticsearch,
when using the `return_errors` option.

Closes #107
[MODEL] Improved the support for returning an array of error elements from `import`

Changed the option name to `return`, to allow future expansion for other return values,
and for improved expressivity:

    Article.import return: 'errors'

Added documentation, tweaked tests.

Related: 107