Using undefined variables in task config produces useless error

[schmichael]

Nomad version

0.9.0 and 0.9.1

Issue

Using an undefined variable for interpolation in a task config stanza produces a useless error message. For example ${FOO_bar} in a Docker dns_servers list would cause the following error:

2019-05-13T15:16:19.293-0700 [ERROR] client.alloc_runner.task_runner: running driver failed: alloc_id=dd29e024-3b05-0575-0deb-745676003cf5 task=redis error="failed to decode driver config: [pos 298]: readContainerLen: Unrecognized descriptor byte: hex: d4, decimal: 212"

The root cause seems to be that undefined variables are defined as Unknown in HCL2:

"labels":[]interface {}{map[string]interface {}{"foo":cty.unknown}}

Reproduction steps

Use an undefined variable in a task config.

Job file (if appropriate)

job "example" {
  datacenters = ["dc1"]

  group "cache" {
    task "redis" {
      driver = "docker"

      config {
        image = "redis:3.2"
        port_map {
          db = 6379
        }
        labels {
          foo = "${INVALID_bar}"
        }
      }

      resources {
        cpu    = 500
        memory = 256
        network {
          mbits = 10
          port "db" {}
        }
      }
    }
  }
}

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