Clarify error messages for admin passwords.

Logs the instance uuid and a more clear error message when errors
occur while setting admin passwords. Fixes LP bug #918502.

Change-Id: I7d698ee87ac147cb5137cb77d0c0297a47507798

nova/compute/manager.py

@@ -913,7 +913,9 @@ def set_admin_password(self, context, instance_uuid, new_pass=None):
 
             if instance_state != expected_state:
                 self._instance_update(context, instance_id, task_state=None)
-                raise exception.Error(_('Instance is not running'))
+                raise exception.Error(_('Failed to set admin password. '
+                                      'Instance %s is not running'),
+                                      instance_ref["uuid"])
             else:
                 try:
                     self.driver.set_admin_password(instance_ref, new_pass)