linux installer: better error message for missing /tmp

linux installer: better error message for missing /tmp

Our linux distribution at my job didn't include a /tmp directory. When ever
I tried to use syslinux, I would get the following error:

./syslinux: No such file or directory

I modified the code to report this:

./syslinux: Cannot access the /tmp/ directory.

This would have helped me a lot rather than having to download the source
and run it through the debugger to figure out that my /tmp/ directory was
missing. I suspect if this ever happens again I'll know exactly what was
wrong, but hopefully it will help someone else.