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/mainpod/operation.rs

@@ -1,11 +1,14 @@
 use std::fmt;
 
-use anyhow::{anyhow, Result};
 use plonky2::field::types::Field;
 use serde::{Deserialize, Serialize};
 
 use crate::{
-    backends::plonky2::{mainpod::Statement, primitives::merkletree::MerkleClaimAndProof},
+    backends::plonky2::{
+        error::{Error, Result},
+        mainpod::Statement,
+        primitives::merkletree::MerkleClaimAndProof,
+    },
     middleware::{self, OperationType, Params, ToFields, F},
 };
 
@@ -78,12 +81,16 @@ impl Operation {
             OperationAux::MerkleProofIndex(i) => crate::middleware::OperationAux::MerkleProof(
                 merkle_proofs
                     .get(i)
-                    .ok_or(anyhow!("Missing Merkle proof index {}", i))?
+                    .ok_or(Error::custom(format!("Missing Merkle proof index {}", i)))?
                     .proof
                     .clone(),
             ),
         };
-        middleware::Operation::op(self.0.clone(), &deref_args, &deref_aux)
+        Ok(middleware::Operation::op(
+            self.0.clone(),
+            &deref_args,
+            &deref_aux,
+        )?)
     }
 }