Use contextual exn-based errors in DEEPWELL

[emmiegit]

This uses the [exn] crate to

as described in

Example error response:

[1001] method 'user_create' failed, at src/api.rs:378:5
|
|-> [1008] failed to create user, at src/endpoints/user.rs:37:10
|
|-> [4101] cannot create user with empty slug, at src/services/user/service.rs:931:9

With the error JSON itself being:

{
    "code": 4101,
    "message": "User slug cannot be empty",
    "data": {
        "call_trace": "[1001] method 'user_create' failed, at src/api.rs:378:5\n|\n|-> [1008] failed to create user, at src/endpoints/user.rs:37:10\n|\n|-> [4101] cannot create user with empty slug, at src/services/user/service.rs:931:9",
        "code_trace": [
            1001,
            1008,
            4101
        ],
        "extra": null
    }
}