I would even define an error to be a situation the code can't handle properly, and those things are usually not under your control (if I rip out a harddisk while some program is running, the developer can't do anything against that - but I would hope that the program fails accordingly and states why it failed).

I'm obviously paraphrasing here, but things like this do happen (USB devices get disconnected, remote services go down etc).

Edit: Obviously, this applies to different code at different levels. Serialization code might fail due to input - but that also is not under the control of the dev writing the serialization logic. Thus, it should fail.