Error when accepting an invitation from a circle you are a member of

[jvir]

When we are already a member of a circle and accept an invitation to the circle we get a status code 400 indicating an error with no further information, in the body of the response it is shown that a code 122 "you are already a member" is returned. It would seem friendlier to give that information and not an error, since the user expects to join a group and would be told that he was already added.