rustjail: Add anyhow context for D-Bus connections

In cases where the D-Bus connection fails, add a little additional context about
the origin of the error.

Fixes: 6561

Signed-off-by: Christophe de Dinechin <dinechin@redhat.com>
Suggested-by: Archana Shinde <archana.m.shinde@intel.com>
Spell-checked-by: Greg Kurz <gkurz@redhat.com>

src/agent/rustjail/src/cgroups/systemd/dbus_client.rs

@@ -36,8 +36,9 @@ pub struct DBusClient {}
 
 impl DBusClient {
     fn build_proxy(&self) -> Result<SystemManager<'static>> {
-        let connection = zbus::blocking::Connection::system()?;
-        let proxy = SystemManager::new(&connection)?;
+        let connection =
+            zbus::blocking::Connection::system().context("Establishing a D-Bus connection")?;
+        let proxy = SystemManager::new(&connection).context("Building a D-Bus proxy manager")?;
         Ok(proxy)
     }
 }
@@ -109,7 +110,9 @@ impl SystemdInterface for DBusClient {
     }
 
     fn unit_exists(&self, unit_name: &str) -> Result<bool> {
-        let proxy = self.build_proxy()?;
+        let proxy = self
+            .build_proxy()
+            .with_context(|| format!("Checking if systemd unit {} exists", unit_name))?;
 
         Ok(proxy.get_unit(unit_name).is_ok())
     }