Relax 404 condition in `Requester` exception handling (#3299)

When using the `Repository.get_branch` method to retrieve a branch that
does not exist in the repository, the error message returned is "branch
not found". This message does not satisfy the original condition
`lc_message == "not found"`, which results in the 404 error not being
handled as intended. In this case, `GithubException.GithubException` is
returned instead of `GithubException.UnknownObjectException`.

This PR updates the condition to check whether the message contains
either "not found" or "no object found", allowing for more flexible and
accurate error handling.

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Co-authored-by: Enrico Minack <github@enrico.minack.dev>