Skip writing uncompressedFile if denied access, and skip reading changelog.txt if it doesn't exist

[Lothrik]

Under certain conditions PoB is denied write access to GetScriptPath(). This generally means the user updates PoB using something other than our built-in updater. In this scenario we cannot write the *.bin TimelessJewelData files to disk, and if we try to do so it breaks quite horribly. Luckily we don't actually need to write those files to disk, so we can just skip it.

See: https://aur.archlinux.org/packages/path-of-building-community-git#comment-873655

Additionally changelog.txt sometimes does not exist. If we attempt to read from it with io.lines before verifying the file actually exists, an error is thrown.

See: https://aur.archlinux.org/packages/path-of-building-community-git#comment-868605

[zao]

I've tested this PR as of 25506a4 on the AUR version of PoB Community by adding a patch file to a checkout of the PKGBUILD.

Prior to the PR I could reproduce the timeless jewel error mentioned in the AUR comments.
After applying the PR patch the problem was gone and timeless jewels in the tree and the jewel finder works as expected.

The problem with changelog.txt I had no natural way to test but it properly shows an empty About box now if the file is artificially removed from the installation directory.

LGTM!