Handle WP_Error on failed guest author creation for method create_guest_author()

[oscarssanchez]

Hi there,

This PR checks if create() returns a WP_Error and displays a warning with the error message https://github.com/Automattic/Co-Authors-Plus/compare/master...oscarssanchez:fix/error-handling-create-guest-author-cli?expand=1#diff-a52979a3ff3a75e65a5166f5fc4a259ef7e243368b5953e587054bbfbada9865R887

Fixes #878

It also changes a couple things I think could make the command more readable (both in code and output in the terminal)

• Changed output from a single line to display success when creating an author, or warning when there's an issue. This could help readability in the terminal in my opinion
• Adds i18n
• Adds documentation to create_guest_author() and create()
• A bit of refactoring to reduce if statements

This could also help with development for #875

Thanks for your feedback!