dispatch: handle empty IOError args

We occasionally throw bare IOError objects (especially in convert),
don't choke on them.

mercurial/dispatch.py

@@ -225,7 +225,8 b' def _runcatch(req):'
                 # it might be anything, for example a string
                 reason = inst.reason
             ui.warn(_("abort: error: %s\n") % reason)
-        elif util.safehasattr(inst, "args") and inst.args[0] == errno.EPIPE:
+        elif (util.safehasattr(inst, "args")
+              and inst.args and inst.args[0] == errno.EPIPE):
             if ui.debugflag:
                 ui.warn(_("broken pipe\n"))
         elif getattr(inst, "strerror", None):