Fetching a transceiver's part info shouldn't fail silently

[bnaecker]

In #104, a customer link failed to come all the way up, because the part-specific overrides for the transceiver's equalization settings were never applied. The underlying issue was likely #111, where we incorrectly marked a transceiver as unsupported too aggressively. Because of that, we failed to fetch the transceiver's MPN, and thus failed to look up the alternate equalization settings.

That's all fine in some sense, because of the link-reconciliation process in Dendrite, that continually tries to apply all the link settings, including these equalization settings. But we have no actual smoking gun proving #111 is the source of the error, because we silently fail in this case. Here, for example:

dendrite/dpd/src/link.rs

Lines 1735 to 1750 in 6ba23e7

	let mpn = {
	let qsfp = switch
	.switch_ports
	.ports
	.get(&port_id)
	.expect("port existence verified in create_link()")
	.lock()
	.await
	.as_qsfp()
	.cloned();
	if let Some(qsfp) = qsfp {
	qsfp_xcvr_mpn(&qsfp).unwrap_or(None)
	} else {
	None
	}
	};

We're fetching the MPN, and later using it, but none of that code logs the fact that the transceiver couldn't be identified. We should do that, to ensure that this case doesn't pass silently in the future.