component AP gateway: raise exceptions on apGet + fix exceptions catching in apGetLocalObject:

new `exceptions.ExternalRequestError` is raised when request done `apGet` has a return
code >= 300.

Fix wrong exception catched in `apGetLocalObject`.

sat/core/exceptions.py

@@ -77,6 +77,10 @@ class DataError(Exception):
     pass
 
 
+class ExternalRequestError(Exception):
+    """Request to third party server failed"""
+
+
 class ConflictError(Exception):
     pass