Remove instances from instance manager if terminated before starting

Ensure that if an instance is terminated before its Propolis zone is started,
it is removed from the instance manager's instance map so that the instance can
be restarted on that sled.

Found via omicron-stress, which is much happier with this fix in place.

Also improve a little bit of tracing.

nexus/src/app/instance.rs

@@ -1133,7 +1133,7 @@ impl super::Nexus {
 
                         error!(
                             self.log,
-                            "failed to set instance to Failed after bad put";
+                            "attempted to set instance to Failed after bad put";
                             "instance_id" => %instance_id,
                             "result" => ?result,
                         );

nexus/src/app/sagas/instance_start.rs

@@ -203,7 +203,8 @@ async fn sis_move_to_starting(
     let instance_id = params.db_instance.id();
     let propolis_id = sagactx.lookup::<Uuid>("propolis_id")?;
     info!(osagactx.log(), "moving instance to Starting state via saga";
-          "instance_id" => %instance_id);
+          "instance_id" => %instance_id,
+          "propolis_id" => %propolis_id);
 
     let opctx = crate::context::op_context_for_saga_action(
         &sagactx,

sled-agent/src/instance.rs

@@ -506,6 +506,10 @@ impl InstanceInner {
                 self.log,
                 "Instance::terminate() called with no running state"
             );
+
+            // Ensure the instance is removed from the instance manager's table
+            // so that a new instance can take its place.
+            self.instance_ticket.terminate();
             return Ok(());
         };