winpgnt.c: fix an outdated error message.

I just spotted it while I was looking through this module anyway. It
was using %.100s to prevent an sprintf buffer overflow, which hasn't
been necessary since I switched everything over to dupprintf, and also
it was printing the integer value of GetLastError() without using my
convenient translation wrapper win_strerror.

windows/winpgnt.c

@@ -1136,8 +1136,8 @@ void spawn_cmd(const char *cmdline, const char *args, int show)
     if (ShellExecute(NULL, _T("open"), cmdline,
 		     args, NULL, show) <= (HINSTANCE) 32) {
 	char *msg;
-	msg = dupprintf("Failed to run \"%.100s\", Error: %d", cmdline,
-			(int)GetLastError());
+	msg = dupprintf("Failed to run \"%s\": %s", cmdline,
+			win_strerror(GetLastError()));
 	MessageBox(NULL, msg, APPNAME, MB_OK | MB_ICONEXCLAMATION);
 	sfree(msg);
     }