don't stop on UserDoesntExist exception on delete

previously, if a user didn't exist in one backend, the _deleteuser
function in ldapcherry would stop on the UserDoesntExist exception,
potentially leaving the user in other backends.
Now it logs it and continues to delete the user in other backend.

ldapcherry/__init__.py

@@ -847,7 +847,14 @@ class LdapCherry(object):
         admin = sess.get(SESSION_KEY, None)
 
         for b in self.backends:
-            self.backends[b].del_user(username)
+            try:
+                self.backends[b].del_user(username)
+            except UserDoesntExist as e:
+                cherrypy.log.error(
+                    msg="User '" + username +
+                        "' didn't exist in backend '" + b + "'",
+                    severity=logging.INFO
+                )
             cherrypy.log.error(
                 msg="user '" + username + "' deleted from backend '" + b + "'",
                 severity=logging.DEBUG