Merge pull request #2902 from andrewbaldwin44/bugfix/login-manager-er…

…ror-message

Add Error Message for Accessing Login Manager without --web-login

locust/web.py

@@ -135,11 +135,6 @@ def __init__(
         # ensures static js files work on Windows
         mimetypes.add_type("application/javascript", ".js")
 
-        if self.web_login:
-            self.login_manager = LoginManager()
-            self.login_manager.init_app(app)
-            self.login_manager.login_view = "login"
-
         if environment.runner:
             self.update_template_args()
         if not delayed_start:
@@ -518,6 +513,16 @@ def update_user():
 
             return {}, 201
 
+    @property
+    def login_manager(self):
+        if self.web_login:
+            login_manager = LoginManager()
+            login_manager.init_app(self.app)
+            login_manager.login_view = "login"
+            return login_manager
+
+        raise AttributeError("The login_manager is only available with --web-login.\n")
+
     def start(self):
         self.greenlet = gevent.spawn(self.start_server)
         self.greenlet.link_exception(greenlet_exception_handler)