/pools/default/trustedCAs errors in different format to the rest of ns_server

Problem
While implementing MB-47172 I notice the json error object returned by the /pools/default/trustedCAs/[id] is in a different format to the rest of the errors returned by ns_server. couchbase-cli has code to handle three different error message formats.

steps to reproduce

1. Setup a cluster
2. Try to delete the self-generated CA:

   curl -uAdministrator:asdasd -XDELETE http://localhost:9000/pools/default/trustedCAs/0

   {"error":"The CA certificate is in use by the following nodes: 172.18.1.75, 127.0.0.1"}%

Note that other endpoints use errors and not error.

Expectation
For the errors messages to be returned in uniformed format across ns_server (ideally the whole of Couchbase Server)

For example one of the following formats could be used instead

["The CA certificate is in use by the following nodes: 172.18.1.75, 127.0.0.1"]

{"errors": ["The CA certificate is in use by the following nodes: 172.18.1.75, 127.0.0.1"]}

{"errors": {0: "The CA certificate is in use by the following nodes: 172.18.1.75, 127.0.0.1"}}

#The last one does not make sense and is shown for completeness

Note
If this not a bug then please let me know and I will change the implementation in couchbase-cli to handle another new error format.