Do not raise during archive size validation when wait thread is finished

What does this MR do?

When performing decompressed archive size validation check using Gitlab::ImportExport::DecompressedArchiveSizeValidator there can be a situation when an undesired exception is raised.

It happens when a validation is performed on very small archives (e.g. 100 bytes). Errno::ESRCH exception is raised when using Process.getpgid(wait_thr[:pid]). The gzip | wc -c command happens so fast that the waiter thread finishes before we can get group process id.

This scenario is likely to happen especially with introduction of granular relations export API (https://docs.gitlab.com/ee/api/group_relations_export.html), where group relation is 1 row and compressed file is tiny (e.g. a group has 1 label). When such group is imported there is a risk of Process.getpgid(wait_thr[:pid]) raising and failing the import.

To test it yourself you can do the following in rails console:

1. Create a tiny .gz file first ( ~100 bytes )
2. Open rails console

command = "gzip -dc /tmp/archive.tar.gz | wc -c"

# This fails since wait_thr finished

stdin, stdout, stderr, wait_thr = Open3.popen3(command, pgroup: true)
pgrp = Process.getpgid(wait_thr[:pid])

=> Errno::ESRCH: No such process

# This succeeds as wait_thr still exists

stdin, stdout, stderr, wait_thr = Open3.popen3(command, pgroup: true); Process.getpgid(wait_thr[:pid])
=> 85288

This MR fixes this issue by catching this exception and setting pgrp to nil.

Mentions #329864 (closed)

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