error or warn on non-json schema inputs

[cameronelliott]

I only get prelude output from cargo-typify with no types, from my .json

Maybe I am making a silly mistake.
xaa.json

I only get the .rs prelude output with no types on this file.

$ cargo typify -o /dev/stdout xaa.json
#![allow(clippy::redundant_closure_call)]
#![allow(clippy::needless_lifetimes)]
#![allow(clippy::match_single_binding)]
#![allow(clippy::clone_on_copy)]

use serde::{Deserialize, Serialize};
$

[ahl]

This doesn't appear to be a json schema file. As I recall, schemars has some mechanism to infer the schema from a json object, but I'm not sure and I haven't used it myself.

[ahl]

I'm leaving this open, but repurposing it: if we load a file that doesn't appear to be JSON schema we should error out or at least warn the user.