fix: add SAML challenge to reauth (#819)

* fix: add SAML challenge to reauth

* add enable_reauth_refresh flag

* address comments

* fix unit test

* address comments

* update

* update

* update

* update

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diff --git a/google/auth/exceptions.py b/google/auth/exceptions.py
index 57f181e..e9e7377 100644
--- a/google/auth/exceptions.py
+++ b/google/auth/exceptions.py
@@ -57,3 +57,7 @@
         super(ReauthFailError, self).__init__(
             "Reauthentication failed. {0}".format(message)
         )
+
+
+class ReauthSamlChallengeFailError(ReauthFailError):
+    """An exception for SAML reauth challenge failures."""