Clarify unsupported architecture upgrade message

[aryeeteygerald_rogers.com]

Summary

Notify users that upgrading from any arch other than
amd64 and i386 is unsupported by freebsd-update.

PR: 204115

Test Plan

None

[emaste]

usr.sbin/freebsd-update/freebsd-update.sh

689–690

FreeBSD shell style places the cases at the same indentation level as the case statement itself:

case $... in
a | b)
        cmd
        ;;
c)
        cmd2
        ;;
esac

[delphij]

I really don't like this approach because it would bar us from retrofit publishing freebsd-update bits (and would create problems for users who have their own freebsd-update instance running). I think it's more appropriate to make fetch_key to provide more meaningful output instead of "Fetching public key from ... failed".

[aryeeteygerald_rogers.com]

• Fix style issues

Also modify error message slightly

[emaste]

Gerald originally suggested an approach similar to what you describe, but I suggested the simpler way here.

Point well taken, new review will come.

[aryeeteygerald_rogers.com]

• Clarify 'mirror failure' message

Update exiting error message to report potential cause and solution.

It would be preferable to exit earlier by searching in some sort of support list
as suggested by @emaste but this should hopefully suffice until then.

[linimon]

I suggest s/can be/can only be/ .

[delphij]

LGTM & Thanks for your work!