Add error in agent init script if the binary wasn't actually downloaded 

A user had Coder configured behind CloudFlare access. The init script downloaded an HTML page from Cloudflare access instead of the Coder binary. The error in the pod logs was

```sh
./coder: 2: Syntax error: newline unexpected
```

Could we add a check to the init script to see if the right file was actually downloaded? Perhaps with a sha256 checksum or some try/catch thingie?