Better exception handling for Mattermost login

mattermost/mattermost.py

@@ -50,7 +50,11 @@ class MattermostCollector(object):
                     'scheme': o.scheme,
                     'port': mm_port
                 })
-                mat_api.login()
+                try:
+                    mat_api.login()
+                except Exception as err:
+                    print("Cannot login to {} : {}".format(instance, err))
+                    continue
                 self.instances.append(
                     {
                         'conn': mat_api,