NameError while processing BlobNotFound by bst 1.6.0

Summary

artifactcache tries to raise BlobNotFound but it's not defined there.

Steps to reproduce

Unknown, this seems like a chained exception.

What is the current bug behavior?

Exception while processing exception

What is the expected correct behavior?

Just the original exception

Relevant logs and/or screenshots

    [00:00:00][bec0953f][ pull:components/json-glib.bst      ] BUG     Pull
        An unhandled exception occured:
    
        Traceback (most recent call last):
          File "/usr/lib/python3.8/site-packages/buildstream/_artifactcache/artifactcache.py", line 770, in pull
          File "/usr/lib/python3.8/site-packages/buildstream/_artifactcache/cascache.py", line 262, in pull
          File "/usr/lib/python3.8/site-packages/buildstream/_artifactcache/cascache.py", line 1062, in init
        OSError: [Errno 24] Too many open files: '/cache-certificate/server.crt'
    
        During handling of the above exception, another exception occurred:
    
        Traceback (most recent call last):
          File "/usr/lib/python3.8/site-packages/buildstream/_scheduler/jobs/job.py", line 384, in _child_action
          File "/usr/lib/python3.8/site-packages/buildstream/_scheduler/jobs/elementjob.py", line 94, in child_process
          File "/usr/lib/python3.8/site-packages/buildstream/_scheduler/queues/pullqueue.py", line 38, in process
          File "/usr/lib/python3.8/site-packages/buildstream/element.py", line 1712, in _pull
          File "/usr/lib/python3.8/site-packages/buildstream/element.py", line 1676, in _pull_strong
          File "/usr/lib/python3.8/site-packages/buildstream/_artifactcache/artifactcache.py", line 778, in pull
        NameError: name 'BlobNotFound' is not defined

Possible fixes

https://gitlab.com/BuildStream/buildstream/-/blob/bst-1/buildstream/_artifactcache/artifactcache.py#L778

Other relevant information

• BuildStream version affected: /milestone %BuildStream_v1.x

---