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/frontend/custom.rs

@@ -1,11 +1,10 @@
 #![allow(unused)]
 use std::{collections::HashMap, fmt, hash as h, iter, iter::zip, sync::Arc};
 
-use anyhow::{anyhow, Result};
 use schemars::JsonSchema;
 
 use crate::{
-    frontend::{AnchoredKey, Statement, StatementArg},
+    frontend::{AnchoredKey, Error, Result, Statement, StatementArg},
     middleware::{
         self, hash_str, CustomPredicate, CustomPredicateBatch, Key, KeyOrWildcard, NativePredicate,
         Params, PodId, Predicate, StatementTmpl, StatementTmplArg, ToFields, Value, Wildcard,
@@ -131,17 +130,17 @@ impl CustomPredicateBatchBuilder {
         sts: &[StatementTmplBuilder],
     ) -> Result<Predicate> {
         if args.len() > params.max_statement_args {
-            return Err(anyhow!(
-                "args.len {} is over the limit {}",
+            return Err(Error::max_length(
+                "args.len".to_string(),
                 args.len(),
-                params.max_statement_args
+                params.max_statement_args,
             ));
         }
         if (args.len() + priv_args.len()) > params.max_custom_predicate_wildcards {
-            return Err(anyhow!(
-                "wildcards.len {} is over the limit {}",
+            return Err(Error::max_length(
+                "wildcards.len".to_string(),
                 args.len() + priv_args.len(),
-                params.max_custom_predicate_wildcards
+                params.max_custom_predicate_wildcards,
             ));
         }