Fix if the identifier provided is one of a blob instead of a commit
    
    Currently if the user provide the hash of a blob instead of the one of a
    commit, we simply crash since the objects are different.
    With this commit, we will raise a 404 error instead of a 500 error.
    
    Fixes an error reported by email
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>