Add custom 500 error handler so app handler errors aren't supressed

Author: fredsa

main.py

@@ -17,3 +17,9 @@ def hello():
 def page_not_found(e):
     """Return a custom 404 error."""
     return 'Sorry, Nothing at this URL.', 404
+
+
+@app.errorhandler(500)
+def page_not_found(e):
+    """Return a custom 500 error."""
+    return 'Sorry, unexpected error: {}'.format(e), 500