user-APIs: forbid setting non-existing attribute

CHANGES.rst

@@ -53,6 +53,9 @@ Specviz2d
 API Changes
 -----------
 
+- User APIs now raise a warning when attempting to set a non-existing attribute to avoid confusion
+  caused by typos, etc. [#2577]
+
 Cubeviz
 ^^^^^^^
 

jdaviz/core/user_api.py

@@ -34,8 +34,10 @@ def __getattr__(self, attr):
         return getattr(exp_obj, 'user_api', exp_obj)
 
     def __setattr__(self, attr, value):
-        if attr in _internal_attrs or attr not in self._expose:
+        if attr in _internal_attrs:
             return super().__setattr__(attr, value)
+        if attr not in self._expose:
+            raise ValueError(f"{attr} is not a valid attribute and cannot be set")
 
         if attr in self._readonly:
             raise AttributeError("cannot set read-only item")