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/primitives/merkletree/error.rs

@@ -0,0 +1,79 @@
+//! tree errors
+
+use std::{backtrace::Backtrace, fmt::Debug};
+
+pub type TreeResult<T, E = TreeError> = core::result::Result<T, E>;
+
+#[derive(Debug, thiserror::Error)]
+pub enum TreeInnerError {
+    #[error("key not found")]
+    KeyNotFound,
+    #[error("key already exists")]
+    KeyExists,
+    #[error("max depth reached")]
+    MaxDepth,
+    #[error("reached empty node, should not have entered")]
+    EmptyNode,
+    #[error("proof of {0} does not verify")]
+    ProofFail(String), // inclusion / exclusion
+    #[error("invalid {0} proof")]
+    InvalidProof(String),
+    #[error("key too short (key length: {0}) for the max_depth: {1}")]
+    TooShortKey(usize, usize),
+}
+
+#[derive(thiserror::Error)]
+pub enum TreeError {
+    #[error("Inner: {inner}\n{backtrace}")]
+    Inner {
+        inner: Box<TreeInnerError>,
+        backtrace: Box<Backtrace>,
+    },
+    #[error("anyhow::Error: {0}")]
+    Anyhow(#[from] anyhow::Error),
+}
+
+impl Debug for TreeError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        std::fmt::Display::fmt(self, f)
+    }
+}
+
+macro_rules! new {
+    ($inner:expr) => {
+        TreeError::Inner {
+            inner: Box::new($inner),
+            backtrace: Box::new(Backtrace::capture()),
+        }
+    };
+}
+use TreeInnerError::*;
+impl TreeError {
+    pub fn inner(&self) -> Option<&TreeInnerError> {
+        match self {
+            Self::Inner { inner, .. } => Some(inner),
+            _ => None,
+        }
+    }
+    pub(crate) fn key_not_found() -> Self {
+        new!(KeyNotFound)
+    }
+    pub(crate) fn key_exists() -> Self {
+        new!(KeyExists)
+    }
+    pub(crate) fn max_depth() -> Self {
+        new!(MaxDepth)
+    }
+    pub(crate) fn empty_node() -> Self {
+        new!(EmptyNode)
+    }
+    pub(crate) fn proof_fail(obj: String) -> Self {
+        new!(ProofFail(obj))
+    }
+    pub(crate) fn invalid_proof(obj: String) -> Self {
+        new!(InvalidProof(obj))
+    }
+    pub(crate) fn too_short_key(depth: usize, max_depth: usize) -> Self {
+        new!(TooShortKey(depth, max_depth))
+    }
+}