Separate the error types

[svartalf]

This battop issue had introduced the case, when the Battery struct instance can represent the missing device. While it is possible now to handle that case (see #29), this kind of error seems to be a recoverable type of error for library users (mostly to the battop at the moment), because it can be handled (for example, by removing the device from the batteries list).

battery::Error can be reworked as an enum with the "recoverable" and "non-recoverable" members, smth like this:

enum Error {
    /// Battery device is missing now
    Gone(io::Error),
    /// Some error happened during the information fetch
    Other(io::Error),
}