uv: improvements to process.binding('uv')

* ensure that UV_... props are constants
* improve error message ... the error message when passing
  in `err >= 0` to `util._errnoException()` was less than
   useful.
* refine uses of process.binding('uv')

PR-URL: https://github.com/nodejs/node/pull/14933


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>