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/middleware/custom.rs

@@ -1,12 +1,11 @@
 use std::{fmt, iter, sync::Arc};
 
-use anyhow::{anyhow, Result};
 use plonky2::field::types::Field;
 use schemars::JsonSchema;
 use serde::{Deserialize, Serialize};
 
 use crate::middleware::{
-    hash_fields, Hash, Key, NativePredicate, Params, ToFields, Value, F, HASH_SIZE,
+    hash_fields, Error, Hash, Key, NativePredicate, Params, Result, ToFields, Value, F, HASH_SIZE,
 };
 
 #[derive(Clone, Debug, PartialEq, Serialize, Deserialize, JsonSchema)]
@@ -217,7 +216,11 @@ impl CustomPredicate {
         args_len: usize,
     ) -> Result<Self> {
         if statements.len() > params.max_custom_predicate_arity {
-            return Err(anyhow!("Custom predicate depends on too many statements"));
+            return Err(Error::max_length(
+                "statements.len".to_string(),
+                statements.len(),
+                params.max_custom_predicate_arity,
+            ));
         }
 
         Ok(Self {
@@ -394,7 +397,6 @@ impl fmt::Display for Predicate {
 mod tests {
     use std::{array, sync::Arc};
 
-    use anyhow::Result;
     use plonky2::field::goldilocks_field::GoldilocksField;
 
     use super::*;