refactor: handle the editor missing scenario with a better message

tutor/commands/config.py

@@ -281,7 +281,7 @@ def edit(context: Context, editor: str, save: bool) -> None:
             f"Missing config file at {config_file}. Have you run 'tutor local launch' yet?"
         )
 
-    open_cmd = None
+    open_cmd = []
     if editor:
         open_cmd = [editor, config_file]
     elif which("open"):  # MacOS & linux distributions that ship `open`. eg., Ubuntu
@@ -292,6 +292,12 @@ def edit(context: Context, editor: str, save: bool) -> None:
         # Calling "start" on a regular file opens it with the default editor.
         # The second argument "" just means "don't give the window a custom title".
         open_cmd = ["start", '""', config_file]
+    else:
+        raise exceptions.TutorError(
+            "Failed to find utility to launch an editor. You can specify the editor "
+            "using the argument -e (e.g., tutor config edit -e vim) or edit "
+            f"{config_file} with the editor of your choice."
+        )
 
     utils.execute(*open_cmd)