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/backends/plonky2/circuits/common.rs

@@ -2,7 +2,6 @@
 
 use std::{array, iter};
 
-use anyhow::Result;
 use plonky2::{
     field::{
         extension::Extendable,
@@ -19,6 +18,7 @@ use plonky2::{
 use crate::{
     backends::plonky2::{
         basetypes::D,
+        error::Result,
         mainpod::{Operation, OperationArg, Statement},
         primitives::merkletree::MerkleClaimAndProofTarget,
     },
@@ -75,7 +75,7 @@ impl StatementArgTarget {
         params: &Params,
         arg: &StatementArg,
     ) -> Result<()> {
-        pw.set_target_arr(&self.elements, &arg.to_fields(params))
+        Ok(pw.set_target_arr(&self.elements, &arg.to_fields(params))?)
     }
 
     fn new(first: ValueTarget, second: ValueTarget) -> Self {