can't delete environment when exception occured in destroy method

[chenyujie]

code abstract as following from murano.common.engine.TaskExecutor:
...
            try:
                LOG.debug('Invoking pre-cleanup hooks')
                self.session.start()
                executor.object_store.cleanup()
            except Exception as e:
                return self.exception_result(e, obj, '<GC>')
            finally:
                LOG.debug('Invoking post-cleanup hooks')
                self.session.finish()
            self._model['ObjectsCopy'] = \
                copy.deepcopy(self._model.get('Objects'))
...

When there is error in service, deleting would always be failed because the exception and the environment keeps the cleanup record, so the environment would be deadlock because it is difficult to fix a non-existing service. However, if we keep running after the exception result, 'ObjectsCopy' would be replaced by 'Object', and the environment could recover normal status. Fix as following:

            except Exception as e:
                self.exception_result(e, obj, '<GC>')