Ensure packages in additional installed repository are not updated

The underlying assumption of the additional installed repository is that those
packages are installed in such a way that they cannot be updated. Prior to this
fix they were still candidates for change. This ensures that they are treated
like platform and root packages and are not candidates for update.
Added FilesystemException to handle errors on local filesystem.
Added earlier permission checks in local filesystem before start download.
Merge pull request #1886 from phansys/local-fs-check

Added local filesystem permission checks for self-update
Merge remote-tracking branch 'simensen/installed-packages'
Clarify code a bit