SoftFail after span closure masks exec failure

[mechie]

If config.SoftFail is called after an exec has finished (e.g. due to the OTel collector returning a non-compliant header), otel-cli will return 0 (or 1, if --fail/OTEL_CLI_FAIL=true), masking any failures in the underlying operation.

Instead, it should probably do something like:

        # exitCode could be a parameter, or come from a helper that fetches it?
	if c.Fail {
		os.Exit(exitCode | 1)  // or just `1`?
	} else {
		os.Exit(exitCode)
	}

Otherwise, otel-cli should probably make clear that its surfacing of the operation's return code is incidental, maybe provide some alternative method of surfacing it (export an OTEL_CLI_LAST_RC?).