Inconsistent error handling on different platforms

[squeek502]

Luv currently assumes all Libuv functions only return negative error codes, but that is not always true. On Windows, Libuv does not always translate its error codes before returning them, meaning that it sometimes returns positive error codes.

One example to illustrate the issue:

• When calling uv_udp_recv_start a second time on the same socket, Libuv will return WSAEALREADY in its Windows implementation, which is the positive integer 10037.
• This never gets translated via uv_translate_sys_error, so when Luv checks for errors in luv_udp_recv_start, this positive error code is ignored
• This means that Windows, the following code will run without an error, while on other platforms, it will error with EALREADY: connection already in progress:

local uv = require('uv')

local socket = uv.new_udp()
assert(uv.udp_bind(socket, "0.0.0.0", 8002))

assert(uv.udp_recv_start(socket, function(err, data, addr, flags)
  print(err, data)
end))

-- this assert fails on Linux, but succeeds on Windows
assert(uv.udp_recv_start(socket, function(err, data, addr, flags)
  print(err, data)
end))

This doesn't seem to be an isolated or rare case, either; there are many places where Libuv returns GetLastError()/WSAGetLastError() directly. Nevermind, this looks like a Libuv bug; other functions that return untranslated error codes are internal Libuv functions that have their errors translated elsewhere.

EDIT: Reported to Libuv: libuv/libuv#1978