Unhandled exception when build reports "bug"

Summary

When building the freedesktop sdk i encountered a stack trace for sdk-integration.bst

Steps to reproduce

Build freedesktop-sdk using the Makefile provided in the git repo using buildstream 1.1.5

What is the current bug behavior?

A stack trace occurs related the cache

What is the expected correct behavior?

No stack trace to occur

Relevant logs and/or screenshots

[00:00:01][f98e9d70][build:sdk-integration.bst           ] BUG     Build

    An unhandled exception occured:

    Traceback (most recent call last):
      File "/home/adds68/.local/lib/python3.7/site-packages/buildstream/_artifactcache/cascache.py", line 95, in extract
        os.rename(checkoutdir, dest)
    OSError: [Errno 39] Directory not empty: '/home/adds68/.cache/buildstream/artifacts/extract/tmpghdzdw4u/freedesktop-sdk/base-iso-codes/f1862ecacbc713a82afc5a789b82c3015a94f49cc03f74095b47dfff90e520b6' -> '/home/adds68/.cache/buildstream/artifacts/extract/freedesktop-sdk/base-iso-codes/1ef40f5f758cacb7454fe65f3c292ccdfe2ac5af1efd256dd6fb249abb141db7'

    During handling of the above exception, another exception occurred:

    Traceback (most recent call last):
      File "/home/adds68/.local/lib/python3.7/site-packages/buildstream/_scheduler/jobs/job.py", line 398, in _child_action
        result = self.child_process()
      File "/home/adds68/.local/lib/python3.7/site-packages/buildstream/_scheduler/jobs/elementjob.py", line 94, in child_process
        return self._action_cb(self._element)
      File "/home/adds68/.local/lib/python3.7/site-packages/buildstream/_scheduler/queues/buildqueue.py", line 34, in process
        element._assemble()
      File "/home/adds68/.local/lib/python3.7/site-packages/buildstream/element.py", line 1463, in _assemble
        collect = self.assemble(sandbox)
      File "/home/adds68/git-repos/freedesktop-sdk/plugins/elements/collect-integration.py", line 47, in assemble
        bstdata = dependency.get_public_data('bst')
      File "/home/adds68/.local/lib/python3.7/site-packages/buildstream/element.py", line 825, in get_public_data
        self.__load_public_data()
      File "/home/adds68/.local/lib/python3.7/site-packages/buildstream/element.py", line 2476, in __load_public_data
        artifact_base, _ = self.__extract()
      File "/home/adds68/.local/lib/python3.7/site-packages/buildstream/element.py", line 2343, in __extract
        return (self.__artifacts.extract(self, key), key)
      File "/home/adds68/.local/lib/python3.7/site-packages/buildstream/_artifactcache/cascache.py", line 102, in extract
        if e.errno not in [os.errno.ENOTEMPTY, os.errno.EEXIST]:
    AttributeError: module 'os' has no attribute 'errno'

Possible fixes

Other relevant information

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