Provide errors as enum#6

It would be helpful if `FfProbeError` was an enum, so one could easily check for the category of error programmatically. My usecase: I would like to log deserialization errors prominently so i can report them here, but the only way to do that is a `str::contains` on the error message. The way i see it, there are currently 3 types of errors that are being wrapped: https://github.com/theduke/ffprobe-rs/blob/88eedce369dd7a2ad29799d40d46ae55fd6f74ba/src/lib.rs#L25-L48 I would propose the following variants: - `Io(std::io::Error)` for errors originating from the ffprobe invocation, for example if the command was not found - `Status(std::process::Output)` for invalid exit codes - `Deserialize(serde_json::Error)` for failure to deserialize I'm willing to provide a PR, just asking for an opinion beforehand.