Check if this is a Pi-hole instead of throwing a random error

[DL6ER]

Thank you for your contribution to the Pi-hole Community!

Please read the comments below to help us consider your Pull Request.

We are all volunteers and completing the process outlined will help us review your commits quicker.

Please make sure you

1. Base your code and PRs against the repositories developmental branch.
2. Sign Off all commits as we enforce the DCO for all contributions
3. Sign all your commits as they must have verified signatures
4. File a pull request for any change that requires changes to our documentation at our documentation repo

---

What does this PR aim to accomplish?:

Today I installed PADD on a device which I thought was a Pi-hole already. The outcome was:

As this error isn't very helpful, this PR checks if this file exists at all before starting PADD.

How does this PR accomplish the above?:

Check if /etc/pihole/setupVars.conf exists. If not, don't (try to) start PADD.

Link documentation PRs if any are needed to support this PR:

N/A

---

By submitting this pull request, I confirm the following:

1. I have read and understood the contributors guide, as well as this entire template. I understand which branch to base my commits and Pull Requests against.
2. I have commented my proposed changes within the code and I have tested my changes.
3. I am willing to help maintain this change if there are issues with it later.
4. It is compatible with the EUPL 1.2 license
5. I have squashed any insignificant commits. (git rebase)
6. I have checked that another pull request for this purpose does not exist.
7. I have considered, and confirmed that this submission will be valuable to others.
8. I accept that this submission may not be used, and the pull request closed at the will of the maintainer.
9. I give this submission freely, and claim no ownership to its content.

---

• I have read the above and my PR is ready for review. Check this box to confirm

[yubiuser]

Do we have /etc/pihole/setupVars.conf on the docker image?
__

Please rebase on development

[DL6ER]

Rebased on latest development