Adds error handling so that if the request to themes.svn.wordpress.org to check theme names fails for some reason, the request fails gracefully and outputs the error to the JS console.

Testing instructions

• Spoof an error when making request to themes.svn.wordpress.org, e.g. add domain to /etc/hosts file or manually return a WP_Error from WP_Theme_Directory::get_theme_names
• See that the with this change, the error is logged to the JS console, rather than being unhandled.