feat: add reauth feature to user credentials (#727)

* feat: add reauth support to oauth2 credentials

* update
diff --git a/google/auth/exceptions.py b/google/auth/exceptions.py
index b6f686b..57f181e 100644
--- a/google/auth/exceptions.py
+++ b/google/auth/exceptions.py
@@ -48,3 +48,12 @@
 class OAuthError(GoogleAuthError):
     """Used to indicate an error occurred during an OAuth related HTTP
     request."""
+
+
+class ReauthFailError(RefreshError):
+    """An exception for when reauth failed."""
+
+    def __init__(self, message=None):
+        super(ReauthFailError, self).__init__(
+            "Reauthentication failed. {0}".format(message)
+        )