improve error message when tempDir does not exist
tested with [0.8.4 recent release](https://forum.nim-lang.org/t/9285) installing on windows and had a very minor issue, thanks for the release!

When installing and temp directory is non existing there is an error reported when copying DLLs.
Error message could probably be improved by checking if temp dir exists and is accessible and alerting the user to fix it (in source `getTempDir` is used which does not do these checks: https://nim-lang.org/docs/os.html#getTempDir).

I am assuming that this issue is not windows specific.

Note that the `AdditionalInfo` which is logged when there is an error copying the DLLs does indeed report the path used so from there it is not very difficult to find the issue that the given path is non existing.
