win: set fallback message if FormatMessage fails
win: set fallback message if FormatMessage fails

FormatMessage can fail, e.g. with ERROR_MUI_FILE_NOT_FOUND. In this case
no error message was set and uv_dlerror wrongly returned "no error".

With this patch the fallback error message "error: <errorno>" is
generated when FormatMessage fails.

PR-URL: https://github.com/libuv/libuv/pull/59
Reviewed-By: Bert Belder <9f2ff180696da386c1be31b4507155616db25dc6@gmail.com>
Reviewed-By: Saúl Ibarra Corretgé <8a7991f6950ea189abf159d6ec98d2fcc13dc5f6@gmail.com>

unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso