feat: add access token credentials (#476)

feat: add access token credentials
diff --git a/google/auth/exceptions.py b/google/auth/exceptions.py
index e034c55..4f66dc2 100644
--- a/google/auth/exceptions.py
+++ b/google/auth/exceptions.py
@@ -28,5 +28,9 @@
     failed."""
 
 
+class UserAccessTokenError(GoogleAuthError):
+    """Used to indicate ``gcloud auth print-access-token`` command failed."""
+
+
 class DefaultCredentialsError(GoogleAuthError):
     """Used to indicate that acquiring default credentials failed."""