[review] Report GetLastError value when DebugActiveProcess fails

[Eli Zaretskii]

> Date: Thu, 14 Nov 2019 11:40:58 -0500
> From: "Tom Tromey (Code Review)" <gerrit@gnutoolchain-gerrit.osci.io>
> 
> --- a/gdb/windows-nat.c
> +++ b/gdb/windows-nat.c
> @@ -1983,7 +1983,8 @@
>  #endif
>  
>    if (!ok)
> -    error (_("Can't attach to process."));
> +    error (_("Can't attach to process %u (error %u)"),
> +	   (unsigned) pid, (unsigned) GetLastError ());

Why not show the text that corresponds to the error code?  A lone code
is not very descriptive, IME.