Restorations fail when using Socket instead of Localhost for DB

[jessecowens]

Describe the bug
Platform InMotion servers use localhost:/var/lib/mysql/mysql.sock for the DB_HOST value, as the team identified the socket to be more performant than TCP/IP localhost. However, when using this host value, restorations fail due to a "No route to host" error.

To Reproduce
Steps to reproduce the behavior:

1. Set the DB_HOST value in wp-config.php to the mysql socket path (reach out to Jesse on Platform i team if you need assistance replicating)
2. Restore a backup file
3. Note the following error in the restoration log:

[message] => Uncaught PDOException: SQLSTATE[HY000] [2002] No route to host in /home/wordpress/doc_root/wp-content/plugins/boldgrid-backup/admin/class-boldgrid-backup-admin-db-import.php:154
Stack trace:
#0 /home/wordpress/doc_root/wp-content/plugins/boldgrid-backup/admin/class-boldgrid-backup-admin-db-import.php(154): PDO->__construct('mysql:host=loca...', 'wordpress', 'arTSbUdobApzWWw')
#1 /home/wordpress/doc_root/wp-content/plugins/boldgrid-backup/admin/class-boldgrid-backup-admin-db-import.php(102): Boldgrid_Backup_Admin_Db_Import->import_lines(Array)
#2 /home/wordpress/doc_root/wp-content/plugins/boldgrid-backup/admin/class-boldgrid-backup-admin-core.php(1317): Boldgrid_Backup_Admin_Db_Import->import('/home/wordpress...')
#3 /home/wordpress/doc_root/wp-content/plugins/boldgrid-backup/admin/class-boldgrid-backup-admin-core.php(2565): Boldgrid_Backup_Admin_Core->restore_database('/home/wordpress...', 'wp_', NULL)
#4 /home/wordpress/doc_root/wp-content/plugins/boldgrid-backup/includes/class-boldgrid-backup-restorer.php(200): Boldgrid_Backup_Admin_Core->restore_archive_file()
#5 /home/wordpress/doc_root/wp-content/plugins/boldgrid-backup/admin/class-boldgrid-backup-admin-core.php(3069): Boldgrid_Backup_Restorer->run()
#6 /home/wordpress/doc_root/wp-includes/class-wp-hook.php(324): Boldgrid_Backup_Admin_Core->wp_ajax_restore('')
#7 /home/wordpress/doc_root/wp-includes/class-wp-hook.php(348): WP_Hook->apply_filters('', Array)
#8 /home/wordpress/doc_root/wp-includes/plugin.php(517): WP_Hook->do_action(Array)
#9 /home/wordpress/doc_root/wp-admin/admin-ajax.php(188): do_action('wp_ajax_boldgri...')
#10 {main}
  thrown
    [file] => /home/wordpress/doc_root/wp-content/plugins/boldgrid-backup/admin/class-boldgrid-backup-admin-db-import.php
    [line] => 154

Expected behavior
The database should be able to restore regardless of whether the localhost or socket is used for DB connections.
Additionally, no failure message is presented to the user when the restoration fails, see #568