permit error if no session_id is available

 * The dropbox provider should error if it can't get a session
   identifier.  A `KeyError` should suffice until we see what an
   actual error condition looks like.

waterbutler/providers/dropbox/provider.py

@@ -348,7 +348,8 @@ async def _create_upload_session(self) -> str:
             expects=(200, ),
             throws=core_exceptions.UploadError
         )
-        return (await resp.json()).get('session_id', None)
+        data = await resp.json()
+        return data['session_id']
 
     async def _upload_parts(self, stream: streams.BaseStream, session_id: str) -> None:
         """Determines the necessary partitioning of the stream (based on max chunk size), and