crostini: Log failure reason when failing to start Crostini container.

Test: Inject a failure and check logs.
Bug: chromium:1048056
Change-Id: Ib48aa40afc15206bf933343091b1c94a55a4ef89
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2035814
Auto-Submit: David Munro <davidmunro@google.com>
Reviewed-by: Nic Hollingum <hollingum@google.com>
Commit-Queue: David Munro <davidmunro@google.com>
Cr-Commit-Position: refs/heads/master@{#738040}

chrome/browser/chromeos/crostini/crostini_manager.cc

@@ -2729,6 +2729,13 @@ void CrostiniManager::OnLxdContainerStarting(
       result = CrostiniResult::UNKNOWN_ERROR;
       break;
   }
+
+  if (result != CrostiniResult::SUCCESS) {
+    LOG(ERROR) << "Failed to start container. VM: " << signal.vm_name()
+               << " container: " << signal.container_name()
+               << " reason: " << signal.failure_reason();
+  }
+
   if (result == CrostiniResult::SUCCESS &&
       !GetContainerInfo(signal.vm_name(), signal.container_name())) {
     VLOG(1) << "Awaiting ContainerStarted signal from Garcon";