Put more information into error message for login failures (#66)

* Put more information into error message for login failures

* Make new error handling code safer

* Disable pylint rules to explicit no-member check

tap_salesforce/salesforce/__init__.py

@@ -312,7 +312,10 @@ def login(self):
             self.instance_url = auth['instance_url']
         except Exception as e:
             error_message = str(e)
-            if resp:
+            if resp is None and hasattr(e, 'response') and e.response is not None: #pylint:disable=no-member
+                resp = e.response #pylint:disable=no-member
+            # NB: requests.models.Response is always falsy here. It is false if status code >= 400
+            if isinstance(resp, requests.models.Response):
                 error_message = error_message + ", Response from Salesforce: {}".format(resp.text)
             raise Exception(error_message) from e
         finally: