Shims panic on an unrecognised command-line flag

`containerd-shimkit` panics when it cannot parse its command line, so a shim
binary given an unrecognised flag aborts with a backtrace notice rather than
saying what was wrong.

`crates/containerd-shimkit/src/sandbox/cli.rs`, in `shim_main`:

```rust
let flags = parse(&os_args[1..]).unwrap();
```

`parse` defines `-v` and `-info`; anything else is an `Err`, and `.unwrap()`
turns that into a panic.

## Reproducing

Measured with a shim built on `containerd-shimkit` 0.1.2:

```
$ containerd-shim-example-v1 --version
thread 'main' panicked at .../containerd-shimkit-0.1.2/src/sandbox/cli.rs:229:38:
called `Result::unwrap()` on an `Err` value:
InvalidArgument("flag provided but not defined: -version")
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
$ echo $?
101
```

`--version` is not special: `-version` and any other undefined flag do the same.
`-v` works and exits 0.

## Why it seems worth fixing

Exit 101 with a backtrace notice reads as a crash in the shim rather than as a
usage error, and `--version` is a natural thing to try on a binary that was just
placed on a node. A shim is also a program containerd execs, so it is worth it
being well behaved about arguments it does not recognise.

The smallest fix is to print the error and exit non-zero instead of unwrapping.
Whether `--version` should also be accepted as a spelling of `-v` is a separate
question.

## Related

#1119 asked for `-info` and is closed. It reports a panic in the same file, at
the logger initialisation rather than at argument parsing, so this is a
different failure with the same shape. That issue notes the class is cosmetic,
which is true here too — the shim works; it is only unhelpful when misused.

Found while smoke-testing a shim after installing it on a node, in a project
built on `containerd-shim-wasm`.

Start in crates/containerd-shimkit/src/sandbox/cli.rs at shim_main and inspect how parse handles arguments other than -v and -info. Reproduce with containerd-shim-example-v1 --version or another undefined flag, then verify that the shim reports the parsing error and exits non-zero without a panic or backtrace notice; --version support is a separate question.

Written by the indexing model from the issue text.