Handle duplicate user creation

[jerkyrs]

Error seen when a user with same handle is attempted to be created again, this should be handled gracefully

" failed, status &{4 running action Action{Contract: auth.iost, ActionName: signUp, Data: ["iost2222", "D9cPjR8cxiZqiSP3XikkNZvLLxiQnsx1... error: Uncaught exception: Error: id existed > iost2222"