[AIRFLOW-370] Create AirflowConfigException in exceptions.py

AirflowConfigException should be created in
the exceptions utility file, not in
configuration.py.

All exceptions should be created in
`exceptions.py`.

https://issues.apache.org/jira/browse/AIRFLOW-370

Closes apache#1689 from jlowin/refactor-exception

airflow/configuration.py

@@ -32,16 +32,15 @@
 from collections import OrderedDict
 from configparser import ConfigParser
 
+from .exceptions import AirflowConfigException
+
 # show Airflow's deprecation warnings
 warnings.filterwarnings(
     action='default', category=DeprecationWarning, module='airflow')
 warnings.filterwarnings(
     action='default', category=PendingDeprecationWarning, module='airflow')
 
 
-class AirflowConfigException(Exception):
-    pass
-
 try:
     from cryptography.fernet import Fernet
 except ImportError:

airflow/exceptions.py

@@ -20,6 +20,10 @@ class AirflowException(Exception):
     pass
 
 
+class AirflowConfigException(AirflowException):
+    pass
+
+
 class AirflowSensorTimeout(AirflowException):
     pass