Avoid use of `anyhow`

[anakrish]

Avoid use of anyhow in Regorus.

Turns out ThisError is not no_std compatible.

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Original description:

According to comparison,

"Use Anyhow if you don't care what error type your functions return, you just want it to be easy. This is common in application code. Use thiserror if you are a library that wants to design your own dedicated error type(s) so that on failures the caller gets exactly the information that you choose".

The errors in Regorus need to be changed to ThisError.

[mkulke]

with the recent addition of no-std targets, we probably cannot use thiserror in the near term. The generic error trait is in std::error::Error and the effort to move it to core:: is ongoing.

There are similar libs with no-std support, but maybe the macro niceties are not required and we can do a bit boilerplate ourselves. looking into that.

[anakrish]

Interesting that anyhow supports no_std but ThisError doesn't (can't).

[mkulke]

this makes sense, because afaiu thiserror is just an elaborate machinery of helper-macros that will desuger into implementations of the std::error::Error trait, while anyhow implements its own error type.

[rijenkii]

thiserror does not require std since v2:
https://github.com/dtolnay/thiserror/releases/tag/2.0.0, dtolnay/thiserror#373