vm-monitor: Log full error on message handling failure (#5604)

There's currently an issue with the vm-monitor on staging that's not
really feasible to debug because the current display impl gives no
context to the errors (just says "failed to downscale").

Logging the full error should help.

For communications with the autoscaler-agent, it's ok to only provide
the outermost cause, because we can cross-reference with the VM logs.
At some point in the future, we may want to change that.

libs/vm_monitor/src/runner.rs

@@ -505,11 +505,14 @@ impl Runner {
                                     Ok(Some(out)) => out,
                                     Ok(None) => continue,
                                     Err(e) => {
-                                        let error = e.to_string();
-                                        warn!(?error, "error handling message");
+                                        // use {:#} for our logging because the display impl only
+                                        // gives the outermost cause, and the debug impl
+                                        // pretty-prints the error, whereas {:#} contains all the
+                                        // causes, but is compact (no newlines).
+                                        warn!(error = format!("{e:#}"), "error handling message");
                                         OutboundMsg::new(
                                             OutboundMsgKind::InternalError {
-                                                error
+                                                error: e.to_string(),
                                             },
                                             message.id
                                         )