test/rgw/multisite: test error handling of forwarded s3:PutBucketPolicy

test/rgw/multisite: test error handling of forwarded s3:PutBucketPolicy

PutBucketPolicy doesn't parse the given policy until after it's
forwarded and applied on the master zone, so add a test that sends a
non-json policy document that will fail to parse

without the fix to rgw_forward_request_to_master(), the InvalidArgument
error Code is still mapped correctly, but the error Message is not
preserved:

>     assert e.response['Error']['Message']
> AssertionError

Signed-off-by: Casey Bodley <cbodley@redhat.com>