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/error.rs

@@ -0,0 +1,63 @@
+use std::{backtrace::Backtrace, fmt::Debug};
+
+use crate::middleware::{DynError, Statement, StatementTmpl};
+
+pub type Result<T, E = Error> = core::result::Result<T, E>;
+
+#[derive(thiserror::Error, Debug)]
+pub enum InnerError {
+    #[error("{0} {1} is over the limit {2}")]
+    MaxLength(String, usize, usize),
+    #[error("{0} doesn't match {1}")]
+    StatementsDontMatch(Statement, StatementTmpl),
+    #[error("invalid arguments to {0} operation")]
+    OpInvalidArgs(String),
+    // Other
+    #[error("{0}")]
+    Custom(String),
+}
+
+#[derive(thiserror::Error)]
+pub enum Error {
+    #[error("Inner: {inner}\n{backtrace}")]
+    Inner {
+        inner: Box<InnerError>,
+        backtrace: Box<Backtrace>,
+    },
+    #[error(transparent)]
+    Infallible(#[from] std::convert::Infallible),
+    #[error(transparent)]
+    Backend(#[from] Box<DynError>),
+    #[error(transparent)]
+    Middleware(#[from] crate::middleware::Error),
+}
+
+impl Debug for Error {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        std::fmt::Display::fmt(self, f)
+    }
+}
+
+macro_rules! new {
+    ($inner:expr) => {
+        Error::Inner {
+            inner: Box::new($inner),
+            backtrace: Box::new(Backtrace::capture()),
+        }
+    };
+}
+use InnerError::*;
+impl Error {
+    pub(crate) fn custom(s: impl Into<String>) -> Self {
+        new!(Custom(s.into()))
+    }
+    pub(crate) fn op_invalid_args(s: String) -> Self {
+        new!(OpInvalidArgs(s))
+    }
+    pub(crate) fn statements_dont_match(s0: Statement, s1: StatementTmpl) -> Self {
+        new!(StatementsDontMatch(s0, s1))
+    }
+    pub(crate) fn max_length(obj: String, found: usize, expect: usize) -> Self {
+        new!(MaxLength(obj, found, expect))
+    }
+}