Check for existence and readability of static leases file before trying to access it

[DL6ER]

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• I have made only one major change in my proposed changes.
• I have commented my proposed changes within the code.
• I have tested my proposed changes, and have included unit tests where possible.
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• I have squashed any insignificant commits. (git rebase)

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What does this PR aim to accomplish?:

Fix wrong error message as mentioned on Discourse (see link below).

How does this PR accomplish the above?:

PHP's suppression of fopen errors does not reliable so we check ourselves for existence + readability of the requested file before trying to open it.

What documentation changes (if any) are needed to support this PR?:

None

[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/converting-dhcp-lease-to-static-php-error/28422/13

[PromoFaux]

Do we want to do some kind of CreateIfNotExists? Or is that handled by something else?

[DL6ER]

No need to create it here as it will be created by the routine that adds the static lease when it does not exist.

https://github.com/pi-hole/AdminLTE/blob/cf0c7193ab3b4ad1d91ab2dff47b1596c009d63d/scripts/pi-hole/php/savesettings.php#L227