[wicketd] replace snafu with thiserror

[sunshowers]

This is currently the only use of snafu in omicron (outside of transitive
deps), and I figure it is simple enough that we can replace it with thiserror,
reducing use of a direct dependency.

This was the first time I'd encountered snafu so I read a bit about it. As far
as I understand, the main benefit of snafu is that it pushes you heavily
towards an error-per-module pattern. However, thiserror does permit that
pattern as well, and in practice it is only a little more verbose than snafu to
do right (map_err vs context, though snafu introduces new types that
aren't in the source code like IoSnafu and ParseSnafu).

I'd love to get your take on it though @andrewjstone.

[andrewjstone]

I forget exactly why I went down this road. I don't see any real benefit to snafu at this point. I agree it's better to keep things standardized with thiserror. Thanks for the fix!