Feature: Add IdentityError to actix-identity crate. (#296)

* Add IdentityError to actix-identity crate.

In order to let crates in the actix web ecosystem interact correctly
with `actix_web::Error`, this commit introduces its own error type,
replacing the previous usage of `anyhow::Error`.

* Mend some clippy warnings on IdentityError.

* Split identity error into more granular versions.

- `MissingIdentityError` occurs whenever we attempt to gather
  information about an identity from a session, and fail.
- `LoginError` occurs whenever we attempt to login via an identity, and
  fail.

* Feedback for identity error implementation.

- `IdentityError` -> `GetIdentityError`
- Move error messages into Display impl where appropriate
- Split `id` and `get_identity` errors into two types
- Implement `source` on custom errors

* Expand identity error types with struct markers.

In order to get a little more future compatibility and reduce
abstraction leaking, this commit introduces some contextual structs to
our identity errors package.

* Improve doc message for SessionExpiryError.

Co-authored-by: Luca Palmieri <20745048+LukeMathWalker@users.noreply.github.com>

* Improve identity error docs and messaging.

Co-authored-by: Luca Palmieri <20745048+LukeMathWalker@users.noreply.github.com>

* Expand LostIdentityError with placeholder.

Adds a placeholder unit struct to the LostIdentityError variant of
GetIdentityError, which should let us expand on that variant with extra
context later if we like.

* Add From coercion for LostIdentityError.

Improve the ergonomics of using the LostIdentityError unit struct.

* Update Cargo.toml

* Update CHANGES.md

* expose identity error module

* fix error impl

Co-authored-by: Luca Palmieri <20745048+LukeMathWalker@users.noreply.github.com>
Co-authored-by: Rob Ede <robjtede@icloud.com>

actix-identity/CHANGES.md

@@ -2,8 +2,11 @@
 
 ## Unreleased - 2022-xx-xx
 
+- Replace use of `anyhow::Error` with specific error types. [#296]
 - Minimum supported Rust version (MSRV) is now 1.59 due to transitive `time` dependency.
 
+[#296]: https://github.com/actix/actix-extras/pull/296
+
 ## 0.5.2 - 2022-07-19
 
 - Fix visit deadline. [#263]