errors: remove unnecessary varargs handling from OutOfBandError In my recent D10465, I moved some code over from scmutil into `OutOfBandError.__init__`. The code was written to deal with an arbitrary number of `message` arguments to the constructor. It turns out that we only ever pass 0 or 1. Given that, let's simplify it. Differential Revision: https://phab.mercurial-scm.org/D10483