[Importer] Consolidate error action names and signatures in WpPostImporter

[dnaber-de]

Currently there are several error actions for errors that may occur during one logical process (importing post, importing file, importing term).

I would like to see them combined to one action per entity:

• wp_import_post_error when something went wrong with the post importing process (including set term relations and meta) The domain should be handled by WP_Errors error code (post, post_meta, post_terms).
• wp_import_file_error. This includes errors on any file system operation as well as HTTP.

The signature should follow the those of the other importers (described in #39 ): First parameter is an instance of WP_Error followed by the import object. Any other contextual data (response values, IDs, etc.) can be passed as data with the WP_Error object.

Subtask of this: #51