fixed error handling for 500 internal server error - version 1.3.9

git-svn-id: https://plugins.svn.wordpress.org/mailmunch/trunk@1022870 b8457f37-d9ea-0310-8a92-e5e31aec5664
fixed error handling for 500 internal server error - version 1.3.9

git-svn-id: https://plugins.svn.wordpress.org/mailmunch/trunk@1022873 b8457f37-d9ea-0310-8a92-e5e31aec5664