Channel error when missing cert

I believe we should not try to close the channel if the certificate wasn't found:

```
[Errno 2] No such file or directory: 'cert_key'
Traceback (most recent call last):
File "./bin/../scripts/uncloud", line 88, in
main(arguments)
File "/home/nico/vcs/uncloud/uncloud/hack/main.py", line 17, in main
vm = VM(config)
File "/home/nico/vcs/uncloud/uncloud/hack/vm.py", line 32, in __init__
self.db = DB(config)
File "/home/nico/vcs/uncloud/uncloud/hack/db.py", line 30, in __init__
self.connect()
File "/home/nico/vcs/uncloud/uncloud/hack/db.py", line 35, in connect
client = etcd3.client(*self.config.etcd_args)
File "/home/nico/vcs/uncloud/venv/lib/python3.8/site-packages/etcd3-0.11.1-py3.8.egg/etcd3/client.py", line 1180, in client
return Etcd3Client(host=host,
File "/home/nico/vcs/uncloud/venv/lib/python3.8/site-packages/etcd3-0.11.1-py3.8.egg/etcd3/client.py", line 113, in __init__
credentials = self._get_secure_creds(
File "/home/nico/vcs/uncloud/venv/lib/python3.8/site-packages/etcd3-0.11.1-py3.8.egg/etcd3/client.py", line 188, in _get_secure_creds
with open(ca_cert, 'rb') as f:
FileNotFoundError: [Errno 2] No such file or directory: 'cert_key'
Exception ignored in:
Traceback (most recent call last):
File "/home/nico/vcs/uncloud/venv/lib/python3.8/site-packages/etcd3-0.11.1-py3.8.egg/etcd3/client.py", line 182, in __del__
self.close()
File "/home/nico/vcs/uncloud/venv/lib/python3.8/site-packages/etcd3-0.11.1-py3.8.egg/etcd3/client.py", line 173, in close
self.channel.close()
AttributeError: 'Etcd3Client' object has no attribute 'channel'
```

Start by reproducing the missing-certificate failure through scripts/uncloud and inspect uncloud/hack/db.py, then trace the etcd3 client initialization shown in the traceback. Confirm the cleanup path when certificate loading fails and verify that the channel is not closed before it exists.

Written by the indexing model from the issue text.