panic on bad JSON for --json-body argument

[davepacheco]

Following #449, I switched to /dev/stdin and found that the command still panics when it fails to parse JSON:

dap@ivanova ~ $ echo '{"source": { "url": "http://catacomb.eng.oxide.computer/media/cloud/debian-11-genericcloud-amd64.raw", block_size: 512 } }' | oxide image create --name my-image --description my-description --os debian --version 11 --json-body /dev/stdin
thread 'tokio-runtime-worker' panicked at 'called `Result::unwrap()` on an `Err` value: Error("key must be a string", line: 1, column: 104)', cli/src/generated_cli.rs:5801:84
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: JoinError::Panic(Id(17), ...)', cli/src/main.rs:59:10