Create anyhow wrapper for unified REST error handling

[whankinsiv]

We currently construct error responses manually in rest_blockfrost for each failure case (parameter parsing, Bech32 conversion, etc.). As suggested by @SupernaviX, introducing a unified AppError type that wraps anyhow::Error and implements From for RESTResponse would simplify error handling across the module. This abstraction will allow REST handlers to use ? error propagation and ensure consistent error responses.

[lowhung]

With thiserror, the define structure representation of errors is really clean whereas anyhow doesn't have much of that at all, so I may opt for that vs using anyhow. IMO it's more suited to what we want to do in the blockfrost module anyways, which is defining structured error types that remove some of the boilerplate / simplify the error handling.

AFAIK anyhow is not transparent, so if a caller wants to know details about the error, they can not simply match it as an enum, whereas this can be done much more easily using thiserror.

This doc is inspiring my implementation.