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/Cargo.toml

@@ -22,7 +22,6 @@ actix-session = "0.7"
 actix-utils = "3"
 actix-web = { version = "4", default-features = false, features = ["cookies", "secure-cookies"] }
 
-anyhow = "1"
 futures-core = "0.3.7"
 serde = { version = "1", features = ["derive"] }
 tracing = { version = "0.1.30", default-features = false, features = ["log"] }