Fix typesystem warnings

[lukaszsamson]

rescue block clauses can only match on Exception.

Fixes warnings on elixir 1.18-dev

    warning: the following clause will never match:

        {:error, err}

    because it attempts to match on the result of:

        e

    which has type:

        %{..., __exception__: true, __struct__: atom()}

    typing violation found at:
    │
 49 │           {:error, err} -> {:error, err}
    │           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    │
    └─ lib/inet_cidr.ex:49: InetCidr.parse_cidr/2

[andyleclair]

I just tried out rc.0 and came to report the same bug. We love the typechecker don't we folks!!