Fix an error with errorCode not being defined so it was throwing a 500 error

[ZachMayry]

I was having an error with this particular part of the code.

[error] #PID<0.609.0> running ExampleWeb.Endpoint (connection 
#PID<0.592.0>, stream id 27) terminated
Server: localhost:4000 (https)
Request: POST /u2f/finish_registration
** (exit) an exception was raised:
    ** (CaseClauseError) no case clause matching: 0
        (u2f_ex) lib/models/errors.ex:11: U2FEx.Errors.get_retval_from_error/1
        (u2f_ex) lib/u2f_ex.ex:70: U2FEx.finish_registration/2        (example) lib/example_web/controllers/u2f_controller.ex:30: ExampleWeb.U2FController.finish_registration/2

Changing the order of these functions fixes the problem.