Error Message and Instructions for Ellens Alien Game (Issue 2992) (#3058)

Fenced the import of new_aliens_collection() function to throw custom error.
Changed instructions to clarify that new_aliens_collection() is meant to be *outside* the Alien() class.
Changed stub file to add a TODO around creating new_aliens_collection() explicitly outsied the Alien() class.

exercises/concept/ellens-alien-game/.docs/instructions.md

@@ -108,16 +108,17 @@ For example:
 2
 ```
 
-## 7. Object Creation
+## 7. Creating a List of Aliens
 
 Ellen loves what you've done so far, but she has one more favor to ask.
-She would like a standalone function to create a `list` of alien objects, given a list of positions (as `tuples`).
+She would like a standalone (_outside the `Alien()` class_) function that creates a `list` of `Alien()` objects, given a list of positions (as `tuples`).
 
 For example:
 
 ```python
 >>> alien_start_positions = [(4, 7), (-1, 0)]
 >>> aliens = new_aliens_collection(alien_start_positions)
+...
 >>> for alien in aliens:
     	print(alien.x_coordinate, alien.y_coordinate)
 (4, 7)

exercises/concept/ellens-alien-game/classes.py

@@ -20,3 +20,6 @@ class Alien:
     """
 
     pass
+
+
+#TODO:  create the new_aliens_collection() function below to call your Alien class with a list of coordinates.

exercises/concept/ellens-alien-game/classes_test.py

@@ -1,12 +1,17 @@
 import unittest
 import pytest
 
-from classes import new_aliens_collection
+
+try:
+    from classes import new_aliens_collection
+except ImportError as err:
+    raise ImportError("We tried to import the new_aliens_collection() function, "
+                      "but could not find it. Did you remember to create it?") from err
 
 try:
     from classes import Alien
 except ImportError as err:
-    raise ImportError("We tried to import the 'Alien' class, but could not find it. "
+    raise ImportError("We tried to import the 'Alien' class from the classes.py file, but could not find it. "
                       "Did you remember to create it?") from err