Fix bare except (#4250)

diff --git a/google/api_core/future/_helpers.py b/google/api_core/future/_helpers.py
index 933d0b8..7c7cc09 100644
--- a/google/api_core/future/_helpers.py
+++ b/google/api_core/future/_helpers.py
@@ -35,5 +35,5 @@
     # We intentionally want to swallow all exceptions.
     try:
         return callback(*args, **kwargs)
-    except:
+    except Exception:
         _LOGGER.exception('Error while executing Future callback.')