feat(npm): Properly handle error from npm-invoked cli

[harmony7]

This PR corrects the case when the fastly command exits with a non-zero exit code when called through the npm wrapper.
Before this fix, the script doesn't run to completion, a stack trace leaks, and the real exit code is not available to the caller.

Example error output when called with non-existent --asdf flag:

ERROR: error parsing arguments: unknown long flag '--asdf'.

node:internal/errors:865
  const err = new Error(message);
              ^

Error: Command failed: /Users/komuro/Library/Application Support/fnm/node-versions/v18.19.1/installation/lib/node_modules/@fastly/cli/node_modules/@fastly/cli-darwin-arm64/fastly --asdf
    at checkExecSyncError (node:child_process:890:11)
    at execFileSync (node:child_process:926:15)
    at file:///Users/komuro/Library/Application%20Support/fnm/node-versions/v18.19.1/installation/lib/node_modules/@fastly/cli/fastly.js:22:1 {
  status: 1,
  signal: null,
  output: [ null, null, null ],
  pid: 22519,
  stdout: null,
  stderr: null
}

Node.js v18.19.1

This happens because the exit code is 1. The wrapper invokes fastly using the child_process.execFileSync() function:

If the process times out or has a non-zero exit code, this method will throw an Error that will include the full result of the underlying child_process.spawnSync().

This PR fixes this by:

• Wrapping the call to execFileSync() with a try / catch.
• Checking the thrown err object for code. If this is set then it means execFileSync() itself failed (couldn't launch, timeout, etc.), so we rethrow the error.
• Checking the thrown err object for status. If this is set then it means the child process has set an exit code, so we set the current process's exit code to the value.

New output:

ERROR: error parsing arguments: unknown long flag '--asdf'.