Under restricted permissions (like inside a podman container) opening
"/proc/self/ns/net" fails with Permission denied. Consequently we cannot
create our bottom NMPNetns instance. That is mostly fine, however we
would log an error message with severity <error>.
Note that test "src/core/platform/tests/test-platform-general" asserts
that no <warn> and <error> messages get logged. Hence, the test will
fail.
That is undesirable. Downgrade the message to <debug> so that the test
passes. Also, it's not clear that this error message is useful here.
Being unable to open a netns fd is fine and not necessarily an error
condition.
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