[Neuxs] Report last successful blueprint realized when blueprint realization fails

[jgallagher]

When blueprint realization succeeds, we report the ID of that blueprint:

omicron/nexus/src/app/background/tasks/blueprint_execution.rs

Lines 112 to 115 in 02303a6

	json!({
	"target_id": blueprint.id.to_string(),
	"needs_saga_recovery": needs_saga_recovery,
	})

When blueprint realization fails, we report the ID of the blueprint that failed, too:

omicron/nexus/src/app/background/tasks/blueprint_execution.rs

Lines 120 to 123 in 02303a6

	json!({
	"target_id": blueprint.id.to_string(),
	"errors": errors
	})

But in the failure case, it would be nice to also report the ID of the last blueprint that we succeeded in realizing. (This might be different on different Nexus instances!)