Address corner case bugs in the check access and get file size helpers in fileio.py

[jonasbardino]

Address a few corner case bugs in the check access and get file size helpers of the fileio.py module as explained in issue #378 and covered by the unit tests in PR #377.

Addresses a couple of old unrelated pylint used-before-assignment warnings to make linting actions happy.

[albu-diku]

I think these changes ought to be covered by a regression test.

[jonasbardino]

I think these changes ought to be covered by a regression test.

I agree and they will be in PR #377 as e.g. with the commented out line with a matching TODO in test_handles_missing_file:
https://github.com/ucphhpc/migrid-sync/pull/377/files#diff-1c61395f8eafbd27d885b5dd81c67d2ae2a90778fe101ad9036499a681920f65R428

The other ones in the check access helpers like in
https://github.com/ucphhpc/migrid-sync/pull/377/files#diff-1c61395f8eafbd27d885b5dd81c67d2ae2a90778fe101ad9036499a681920f65R1089
can't be triggered with the existing unit tests in CI as long as they by default run as the super-user inside docker and render permission checks useless. So it currently only shows for make unittest. There's a note about it in the description of #377.