implemented editing of list credentials for admin/owner, fixed bug:

if login fails due to wrong auth, it is no longer repeated. instead, the list is disabeld

src/main/java/de/srsoftware/widerhall/mail/ProblemListener.java

@@ -0,0 +1,7 @@
+package de.srsoftware.widerhall.mail;
+
+import javax.mail.MessagingException;
+
+public interface ProblemListener {
+    public void onImapException(MessagingException e);
+}