qa/multisite: fix test_bucket_remove false failure on bucket deletion check

qa/multisite: fix test_bucket_remove false failure on bucket deletion check

get_bucket() was silently returning None for missing buckets instead of
raising, causing check_all_buckets_dont_exist() to always report buckets
as still present after deletion. fix get_bucket() to propagate ClientError

also, replace bare exception with specific ClientError handling
and logging in check_all_buckets_exist and check_all_buckets_dont_exist

Signed-off-by: Shilpa Jagannath <smanjara@redhat.com>