Base behaviour and shared shape for all errors raised or returned by Weavr.

Every error struct in this library implements Exception (so it can be raised with raise/1 when using the ! function variants) and carries a consistent set of fields so callers can pattern-match generically:

case Weavr.Accounts.get(client, id) do
  {:ok, account} -> account
  {:error, %Weavr.Error.API{status: 404}} -> nil
  {:error, %Weavr.Error.API{status: 401}} -> reauthenticate()
  {:error, %Weavr.Error.Network{}} -> retry_later()
end