Spawn should fail if spawning a viewer that is incompatible

[jleibs]

• Send TCP protocol header to ignore non-rerun clients #6253 changed the Rerun protocol version for the first time.

If using C++/Rust, it's possible for the SDK to spawn an old viewer that can't handle the new protocol.

This results in the viewer rejecting the incoming connection with a warning:

[2024-05-23T21:46:04Z WARN re_sdk_comms::server] Closing connection to client at 127.0.0.1:53540: SDK client is using a newer protocol version (1) than the SDK server (0)

Although spawn generates an error message such as

    ⚠ The version of the Rerun Viewer available on your PATH does not match the version of your Rerun SDK ⚠

    Rerun does not make any kind of backwards/forwards compatibility guarantee yet: this can lead to (subtle) bugs.

    Rerun Viewer: v0.15.0 (executable: "rerun")
    Rerun SDK: v0.16.0

it doesn't cause the spawning application to terminate. This means the warning message can be lost and the place where the failure happens is less obvious.

At a minimum we should have spawn return an error code to the user so they can determine whether to fail out or keep running.