Only compare OS versions if dig was successful

[rdwebdesign]

What does this PR aim to accomplish?:

On the current Debug Log code, if the dig command fails it is impossible to check if the OS is supported, but the log prints an error message saying the OS is unsupported (even when that's not true), like this:

*** [ DIAGNOSING ]: Operating system
[i] dig return code:  10
[i] dig response:  dig: couldn't get address for 'ns1.pi-hole.net': failure
[✗] Distro:  Debian
[✗] Error: Debian is not a supported distro (https://docs.pi-hole.net/main/prerequisites/)

How does this PR accomplish the above?:

Modifying the os_check() function to compare versions only if dig returns without error.
Also, adding a new error message for this case:

*** [ DIAGNOSING ]: Operating system
[i] dig return code:  10
[i] dig response:  dig: couldn't get address for 'ns1.pi-hole.net': failure
[✗] Distro:  Debian
[✗] Error:  dig command failed - Unable to check OS

Link documentation PRs if any are needed to support this PR:
none

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[PromoFaux]

Note: This is a redo of #5050 - in which things went very wrong somehow.

[pralor-bot]

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

https://discourse.pi-hole.net/t/pi-hole-ftl-v5-20-1-web-v5-18-1-and-core-v5-15-released/60495/1