Introduce custom actions for OK pressed on alerts

- A block that allows for custom actions if the OK button is pressed
Fix error handling and use WPError for EditSite

- Use the OK block for sending the user to their site

- Fixes the 405 error handling as it was always −1011 instead of 405. Tested with Charles proxy.
Fix error handling and use WPError for WPTableVC

- Use the OK block for sending the user to their site for XML RPC error

- Fixes the 405 error handling as it was sending −1011 instead of 405. Tested with Charles proxy.