Limit use of anyhow

[Gnurou]

Anyhow should only be used when the returned errors are not well defined. For most cases, thiserror is a better and lighter fit. This issue is to track the removal of anyhow from the codec and decoder code.

[dwlsalmeida]

@Gnurou IIRC there was the push at the beginning to do the exact opposite, i.e.: to go from custom error types to anyhow?

[Gnurou]

Was there? I don't know what the rationale was, but in my experience thiserror forces a stronger discipline wrt. how we handle errors (and may end up being more lightweight), so I'd like to consider using it for at least the lowest layers. I'm happy to consider counter-arguments though.