Make uv_err_name() and uv_strerror() return a dynamically allocated
string when the error code is not recognized.
It leaks a few bytes of memory but that can't be helped. Asserting
and aborting is, in my opinion, much less helpful.
Fixes: https://github.com/nodejs/node/issues/63
PR-URL: https://github.com/libuv/libuv/pull/467
Reviewed-By: Saúl Ibarra Corretgé <saghul@gmail.com>