cmd/eval: add support for non-zero exit on undefined result

[tsandall]

Users integrating OPA into pipelines or other automation systems use the opa eval subcommand to execute policies. In these situations, it may be useful if the opa eval subcommand exited with a non-zero code to indicate an undefined result or runtime error. E.g., the subcommand could exit with:

• 0 to indicate success
• 1 to indicate undefined
• 2 to indicate errors

Example interaction:

$ opa eval --fail '1 = 1'
{
  "result": [
    {
      "expressions": [
        {
          "value": true,
          "text": "1 = 1",
          "location": {
            "row": 1,
            "col": 1
          }
        }
      ]
    }
  ]
}
$ echo $?
0
$ opa eval --fail '1 = 0'
{}
$ echo $?
1

The flag name --fail is copied from curl.

The changes to add support for this can be added to cmd/eval.go. If len(result.Result) == 0 at the end of the eval function, then the result is undefined. The eval function can modified to check for this case and return an error accordingly.