Some small exception cleanups

Whitespace change and compare directly
to none since user exceptions may have
overloaded the boolean operator and
avoid checking the exception classes
if the input is empty in the first place.

Change-Id: I426741bbc23181e96084383eaf77daa1d612ab59

taskflow/exceptions.py

@@ -101,7 +101,7 @@ class WrappedFailure(TaskFlowException):
         self._causes = []
         for cause in causes:
             if cause.check(type(self)) and cause.exception:
-                # flatten wrapped failures
+                # NOTE(imelnikov): flatten wrapped failures
                 self._causes.extend(cause.exception)
             else:
                 self._causes.append(cause)
@@ -122,11 +122,13 @@ class WrappedFailure(TaskFlowException):
         of given type, the corresponding argument is returned. Else,
         None is returned.
         """
+        if not exc_classes:
+            return None
         for cause in self:
             result = cause.check(*exc_classes)
-            if result:
+            if result is not None:
                 return result
         return None
 
     def __str__(self):
-        return 'Wrapped Failure: %s' % self._causes
+        return 'WrappedFailure: %s' % self._causes