testsys: convert error from anyhow to snafu

[ecpullen]

A little late to comment on this, but I just realized that in all the Bottlerocket repo, we don't use anyhow and instead we use snafu. I think we should be consistent and move at least this part of testsys to snafu. I'm not blocking on this though but an issue should be good.

Originally posted by @arnaldo2792 in https://github.com/bottlerocket-os/bottlerocket/pull/2273#discussion_r924744267