Nomad should throw an error if script check is used in a group-level service and no task argument is set.

### Nomad version
Nomad v0.12.0-beta2 (5b80d4e638f1a27eee3ca245f8babb115e4c098d)

### Operating system and Environment details
Ubuntu 18.04 on AMD64
Consul v1.8.0

### Issue
Script checks on group-level services require the `task` argument to be set per the [docs](https://www.nomadproject.io/docs/job-specification/service#task-2). However, if no task name is provided, Nomad does not inform the user, and the allocation will just quietly become unhealthy after the `progress_deadline` has passed.

Instead, Nomad should inform the user of a missing task name as part of the jobspec validation/planning. This could be similar to the task specification for a group-level service with connect native enabled, where if no task name (or an invalid task name) is given, an error is thrown:
`Plan Error
error in job mutator connect: native task named by group_name->service_name does not exist`

### Reproduction steps
Run the below job

### Job file (if appropriate)

```hcl
job "ubuntu" {
datacenters = ["dc1"]
group "ubuntu" {
task "ubuntu" {
config {
image = "ubuntu:latest"
command = "bash"
tty = true
}
driver = "docker"
}

network {
mode = "bridge"
mbits = 10
}

service {
name = "test-service"
port = 8080
address_mode = "auto"
connect {
sidecar_service {
}
}

check {
name = "test-service-health"
type = "script"
command = "ls"
interval = "5s"
timeout = "3s"
}
}
}
}
```

With that jobspec, the allocation will become unhealthy. If the line `task = "ubuntu"` is added to the check stanza, the check passes and the allocation becomes healthy as expected.

Start by reproducing the supplied jobspec and comparing planning behavior with and without task = "ubuntu" in the script check. Read the jobspec validation/planning path and the existing connect native task validation mentioned in the issue; done means a missing task produces a plan error while a valid task still allows the check to pass.

Written by the indexing model from the issue text.