Avoid hanging artifact cache tests

Description

We use multiprocessing to run artifact cache push/pull tests. If something breaks inside BuildStream while doing that, the exception is discarded and the test waits forever for queue.get to complete.

Changes proposed in this merge request:

• Add a wrapper to functions in pull.py which will add an entry to the queue if an exception occurs
• Catch exceptions in ArtifactShare.run which adds an entry to the queue.

Note that in some cases the thing placed on the queue will cause an error if the code expects a tuple. This can already happen in the existing code (see _test_push_tree) and is still better than a hang.

---