This fixes https://emqx.atlassian.net/browse/EMQX-8648. The issue
described in `EMQX-8648` is that when deleting a non-existing bridge the
server gives a success response. See below:
```
curl --head -u admin:public2 -X 'DELETE' 'http://localhost:18083/api/v5/bridges/webhook:i_do_not_exist'
HTTP/1.1 204 No Content
date: Tue, 03 Jan 2023 16:59:01 GMT
server: Cowboy
```
After the fix, deleting a non existing bridge will give the following
response:
```
HTTP/1.1 404 Not Found
content-length: 49
content-type: application/json
date: Thu, 05 Jan 2023 12:40:35 GMT
server: Cowboy
```
Closes: EMQX-8648
b59c4c34c5