migrate from anyhow to thiserror (#197)

* migrate from anyhow to thiserror (#190). pending polish error msgs

* Add backtrace and compartmentalize errors

- Include backtraces in the errors we generate.  To get this we can't
  just return a literal enum, because the backtrace requires a call.
- Related to the previous point: add methods to create errors so
  we can include the backtrace conveniently without changing too much
  the syntax.  So instead of `Err(Error::KeyNotFound(key))` (literal
  enum) it will be `Err(Error::key_not_found(key))` (method call)
- Each error should be local to its scope, and each scope should
  only return its own error.
  - The merkle tree should return `TreeError` and not Error
  - The middleware should return `MiddlewareError` and not Error
- With a global Error we can't easily include backend/frontend types in
  the error fields, so declare a `BackendError` and a `FrontendError`
  and follow the pattern from the previous point
- The Pod traits should be able to return backend errors and will be
  used in the frontend; for that we change them to use trait object
  Error: `dyn std::error::Error`

* fix error

* apply suggestions from @arnaucube

* rename XError and XResult to Error and Result

* reorg signature

* make frontend custom error more ergonomic

* remove unnecessary feature

---------

Co-authored-by: Eduard S. <eduardsanou@posteo.net>

src/examples/custom.rs

@@ -1,10 +1,9 @@
 use std::sync::Arc;
 
-use anyhow::Result;
 use StatementTmplBuilder as STB;
 
 use crate::{
-    frontend::{key, literal, CustomPredicateBatchBuilder, StatementTmplBuilder},
+    frontend::{key, literal, CustomPredicateBatchBuilder, Result, StatementTmplBuilder},
     middleware::{
         CustomPredicateBatch, CustomPredicateRef, NativePredicate as NP, Params, PodType,
         Predicate, KEY_SIGNER, KEY_TYPE,

src/examples/mod.rs

@@ -2,12 +2,11 @@ pub mod custom;
 
 use std::collections::HashSet;
 
-use anyhow::Result;
 use custom::{eth_dos_batch, eth_friend_batch};
 
 use crate::{
     backends::plonky2::mock::signedpod::MockSigner,
-    frontend::{MainPodBuilder, SignedPod, SignedPodBuilder},
+    frontend::{MainPodBuilder, Result, SignedPod, SignedPodBuilder},
     middleware::{
         containers::Set, CustomPredicateRef, Params, PodType, Statement, TypedValue, Value,
         KEY_SIGNER, KEY_TYPE,