BUG: Better error handling when invalid API keys are entered.

git-svn-id: https://plugins.svn.wordpress.org/pmpro-mailchimp/trunk@974326 b8457f37-d9ea-0310-8a92-e5e31aec5664
Handling the blank case again. Sneaky update.

git-svn-id: https://plugins.svn.wordpress.org/pmpro-mailchimp/trunk@974327 b8457f37-d9ea-0310-8a92-e5e31aec5664