Nova compute: add in log exception to help debug failures

When the system is under load, there can be boot failures. One
exception seen in the log file is:

''NoneType' object has no attribute 'status_code''

There is no log trace which helps pinpoint this client call.
Note that this is only one example.

We now add in a LOG.exception to have the stack trace.

Related-Bug: #1850682

Change-Id: I2ab56005db363f81eecf07c3db62c267ba7f8ca9
(cherry picked from commit 9062fd3923)

nova/compute/manager.py

@@ -2506,6 +2506,8 @@ class ComputeManager(manager.Manager):
             raise exception.BuildAbortException(instance_uuid=instance.uuid,
                     reason=e.format_message())
         except Exception as e:
+            LOG.exception('Failed to build and run instance',
+                          instance=instance)
             self._notify_about_instance_usage(context, instance,
                     'create.error', fault=e)
             tb = traceback.format_exc()