invalid update to service definition does not fail during registration

### Nomad version
0.12.5

### Operating system and Environment details
linux

### Issue
Update to the service stanza with an invalid configuration does not fail during deployment, as it should do normally.
The job file below normally will trigger an error during deployment saying:
```
unable to get address for service "example-service": invalid port "8080": port label not found
```
However, the validation failed to detect the wrong `check` stanza if the job file is deployed first without check, then adding the check in a subsequent deployment.

### Reproduction steps

1. remove the check section from the job file below and deploy
2. add the check section back and deploy
3. deploy finishes "successfully" however no check is registered as the check is invalid.

### Job file (if appropriate)

Click this to expand

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

group "example-service-api-group" {
service {
name = "example-service"
port = "8080"

check {
type = "http"
path = "/"
interval = "5s"
timeout = "1s"
}
}

task "example-fake" {
driver = "docker"

config {
image = "nicholasjackson/fake-service:v0.12.0"
}

env {
LISTEN_ADDR = "0.0.0.0:8080"
}
}
}
}
```

Start by reproducing the behavior with the supplied Nomad job: deploy it without the check stanza, then add the check and deploy again. Compare validation during initial registration with validation during the update; done means the invalid service update fails with the reported port-label error instead of completing without registering the check.

Written by the indexing model from the issue text.