[BUG] --json outputs errors to stdout instead of stderr

[raineorshine]

Current Behavior:

Running npm ls --json with missing dependencies outputs the error to stdout instead of stderr. This creates two top-level JSON objects, making it unparseable.

This is a regression from v6.

May be related to: #1109

Expected Behavior:

Normal output should print to stdout, errors should print to stderr.

i.e.

stdout:

{
  "name": "temp",
  "problems": [
    "missing: foo@1.0.0, required by temp@"
  ],
  "dependencies": {
    "foo": {
      "required": "1.0.0",
      "missing": true,
      "problems": [
        "missing: foo@1.0.0, required by temp@"
      ]
    }
  }
}

stderr:

{
  "error": {
    "code": "ELSPROBLEMS",
    "summary": "missing: foo@1.0.0, required by temp@",
    "detail": ""
  }
}

Steps To Reproduce:

For the life of me I can't figure out why this test passes, as it clearly asserts the expected stdout.

That said, I can reproduce the issue running the cli manually on a simple package.json:

$ mkdir issue-2150 && cd issue-2150
$ echo '{ "dependencies": { "foo": "1.0.0" } }' > package.json
$ npm ls --json 2>/dev/null

2>/dev/null makes it clear that the following output is all on stdout:

{
  "name": "temp",
  "problems": [
    "missing: foo@1.0.0, required by temp@"
  ],
  "dependencies": {
    "foo": {
      "required": "1.0.0",
      "missing": true,
      "problems": [
        "missing: foo@1.0.0, required by temp@"
      ]
    }
  }
}
{
  "error": {
    "code": "ELSPROBLEMS",
    "summary": "missing: foo@1.0.0, required by temp@",
    "detail": ""
  }
}

Environment:

OS: 10.14.6
Node: 15.1.0
npm: 7.0.9