Fix how api_core.operation populates exception errors (#4231)

diff --git a/google/api_core/operation.py b/google/api_core/operation.py
index 2136d95..1e64542 100644
--- a/google/api_core/operation.py
+++ b/google/api_core/operation.py
@@ -110,7 +110,7 @@
             elif self._operation.HasField('error'):
                 exception = exceptions.GoogleAPICallError(
                     self._operation.error.message,
-                    errors=(self._operation.error),
+                    errors=(self._operation.error,),
                     response=self._operation)
                 self.set_exception(exception)
             else: