fs_event_wrap: remove SetErrno
node_stat_watcher: remove SetErrno
signal_wrap: remove SetErrno
timer_wrap: fix number conversion buglets

* uv_timer_get_repeat() returns uint64_t, not int64_t. Ensure that the
  return value fits in a JS number.

* uv_timer_start() and uv_timer_set_repeat() take uint64_t, not int64_t.
  Ensure that values are >= 0 to avoid wrap-around to values >= 2^63.
timer_wrap: add error checking, return undefined

The uv_timer_*() functions don't fail. Add sanity check asserts and
change the return value of the binding functions to Undefined().

lib/timers.js doesn't check return values anyway so there is not much
point in returning anything besides Undefined().