VaultPress: Add a yellow warning to the "Settings" page if connection fails pre-flight checks

#### Steps to reproduce the issue
- Add an SSH, SFTP, or FTP connection on the VaultPress settings page
- Connection established (i.e., credentials are valid)
- User account not configured correctly (i.e., can't drop/delete/update files) and fails pre-flight checks
- Account is saved as "green" (functional) when it really is only useful for backups (failed pre-flight = can't restore)

It'd be useful if we presented a more noticeable warning to a user if the pre-flight fails, such as saving the credentials with a "yellow" check mark, or sending the user an e-mail explaining how this will impact them if they need to restore via VaultPress.

Start at the VaultPress Settings page and trace how SSH, SFTP, and FTP connections are saved after pre-flight checks. The work is done when a failed pre-flight check is visibly distinguished from a fully functional connection and the user is warned that restore operations may fail.

Written by the indexing model from the issue text.