Several improvements to the Content Diff

[iuravic]

Continuing work from PR #141 here are a few improvements we should add to the Content Diff structure:

• let's take an example of this current error message - Error, could not insert term_relationship for live post/object_id=14077 (new post_id=310) because term_taxonomy_id=990 is not found in live DB -- it exists in live term_relationships, but not in live term_taxonomy table. This should be improved so that in case where a Term is missing, we shouldn't be bringing over term_taxonomy_ids at all.
• change this error message from and "Error" to a "warning" wording and warn that a specific taxonomy/term hasn't been brought over

Additions:

• update_featured_images needs to be refactored better -- the loop and the logging should go to the command class, and just pure "business logic" should be left in this command
• same goes for update_blocks_ids
• refactor get_existing_post_by_live_id method from the command class into the CD logic class