Check permissions of important files before starting the installer

This was prompted by https://self-hosted.getsentry.net/organizations/self-hosted/issues/235/?project=3&query=is%3Aunresolved&referrer=issue-stream, but I believe that we have seen this before with other errors. I think we should probably start collecting files we know we should check before running:

- `enhance-image.sh` should be executable
- `install/*.sh` should be executable
- make sure we can read config files

(Please comment or edit this comment with more things we should check for)