fix: add exception context to GenericGBQExceptions (#629)

Co-authored-by: Tim Swast <swast@google.com>

Author: quoimec

packages/pandas-gbq/pandas_gbq/gbq.py

@@ -393,7 +393,7 @@ def process_http_error(ex):
             error_message = ex.errors[0]["message"]
             raise TableCreationError(f"Reason: {error_message}")
         else:
-            raise GenericGBQException("Reason: {0}".format(ex))
+            raise GenericGBQException("Reason: {0}".format(ex)) from ex
 
     def download_table(
         self,