improv: global modeler error type, finer-grained errors, using thiserror

[flxzt]

This refactors the errors into a separate module, restructures them a bit and introduces a crate-wide ModelerError intended to be returned in all results in the crate - currently it only holds the ElementError variant, but allows for future additions.
All error types are deriving the thiserror Error trait, improving their messages and general handling.

[Doublonmousse]

That looks good.
Having errors that can be nested like this is cool !

I think the description of the repository (the about) should be changed because the repository is not a binding anymore.

I'll have to update the rnote PR after this is merged to take into account the new error types

[flxzt]

Having errors that can be nested like this is cool !

Yeah and thiserror also automatically derives a From impl when a src field is annotated with #[from].

I think the description of the repository (the about) should be changed because the repository is not a binding anymore.

Done.

I also fixed some typos found throughout the crate