Update app.py

Added error handling.

app.py

@@ -50,7 +50,9 @@ def extract_images(url):
 
 # Call Function to communicate with my Wit.ai app
 def wit_ai_response(message):
-    access_token = 'WIT_AI_ACCESS_TOKEN'
+    access_token = os.getenv('WIT_AI_ACCESS_TOKEN')
+    if not access_token:
+        return 'Error: Access token is not set.'
     headers = {
         'Authorization': f'Bearer {access_token}'
     }