job.files: raise FileNotFoundError rather than AttributeError or KeyError

[jan-janssen]

No description provided.

[jan-janssen]

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ERROR: test_job_files (flex.test_executablecontainer.TestExecutableContainer)
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Traceback (most recent call last):
  File "/home/runner/work/pyiron_base/pyiron_base/pyiron_base/jobs/job/extension/files.py", line 104, in __getattr__
    return self[self._get_file_dict()[item]]
KeyError: '__dict__'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/runner/work/pyiron_base/pyiron_base/tests/flex/test_executablecontainer.py", line 97, in test_job_files
    self.assertTrue(file in dir(job.files))
  File "/home/runner/work/pyiron_base/pyiron_base/pyiron_base/jobs/job/extension/files.py", line 58, in __dir__
    return list(self._get_file_dict().keys()) + super().__dir__()
  File "/home/runner/work/pyiron_base/pyiron_base/pyiron_base/jobs/job/extension/files.py", line 106, in __getattr__
    raise FileNotFoundError(item)
FileNotFoundError: __dict__

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[niklassiemer]

LGTM

[pmrv]

Some nits, but generally lgtm.