"multipass <command> --format=json" prints plaintext when the command fails

Describe the bug multipass <command> --format=json does not output JSON when the command fails.

To Reproduce

1. multipass info nonexisting-instance --format=json
2. Observe info failed: The following errors occurred: instance "nonexisting-instance" does not exist is printed

Expected behavior The error message should be encapsulated in JSON, like

{
    "error_msg": "info failed: The following errors occurred: instance \"nonexisting-instance\" does not exist`"
    "exit_code": 1
}