tests/functional/asset: Fail assert fetch when retries are exceeded

tests/functional/asset: Fail assert fetch when retries are exceeded

Currently the fetch code does not fail gracefully when retry limit is
exceeded, it just falls through the loop with no file, which ends up
hitting other errors.

Add a check for non-existing file, which indicates the retry limit was
exceeded.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Message-ID: <20250312130002.945508-2-npiggin@gmail.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>