[AIRFLOW-1639] Fix Fernet error handling

When the encrypted string cannot be decryted using
Fernet for some
reason, an error will be thrown. In Python 3.6.2
the .message attr
is not available. By casting the ValueError to a
string, the message
will be extracted from the Error.

Closes apache#2629 from Fokko/AIRFLOW-1639-fix-error-
handling

(cherry picked from commit aedf8de)
Signed-off-by: Bolke de Bruin <[email protected]>

airflow/models.py

@@ -101,8 +101,7 @@ def get_fernet():
     try:
         return Fernet(configuration.get('core', 'FERNET_KEY').encode('utf-8'))
     except ValueError as ve:
-        raise AirflowException("Could not create Fernet object: {}"
-                               .format(ve.message))
+        raise AirflowException("Could not create Fernet object: {}".format(ve))
 
 
 if 'mysql' in settings.SQL_ALCHEMY_CONN: