Soften DNSSEC error message for external DNS users

[stsievert]

What does this PR implement?
This adds some information about the DNSSEC warning for external/custom DNS users.

I spent some time re-enabling DNSSEC through my registrar because of the strong wording in the warning ("resolve this issue immediately.") See #404 (comment) for more detail.

[JoshData]

"Custom DNS" is used in this project to refer to user-specified DNS records that the box serves when serving DNS. Please revise the comments to say "when not using this box for DNS (external DNS)".

[stsievert]

Updated. Should this use output.print_warning instead of output.print_error?

[JoshData]

No. Many of the status checks are not applicable when existing external DNS, but they are all errors because the project is designed to be correct when used following the instructions.

[stsievert]

What else needs to be done before merge? I cleaned up tabs/spaces again.

[JoshData]

It still says "custom DNS' in the last line.

[stsievert]

Whoops, fixed. Thanks.

[JoshData]

Apologies, there are conflicts now that need to be resolved.

[stsievert]

I've fixed the merge conflicts.

I am unsure if 178c587 resolves this issue; if a DS record is set for any domains that do not have DNS hosted by the box, does the error message still appear?

178c587 includes a note that the error message is shown "[i]f a DS record is set for any of your domain names that have DNS hosted on your box."