Client-Server Versions Mismatch, Not Apparent in Client Failure Messaging

### Nomad version

On client:
```
-- rottaway@stg-ci-agent52:~ $ nomad -version
Nomad v0.5.0
```

On server:
```
-- rottaway@stg-clustermgr-master08:~ $ nomad version
Nomad v0.5.5
```

### Operating system and Environment details

Ubuntu 16.04, EC2 client=m3.large, server=m3.xlarge

### Issue

Very misleading message due to client being on v.5 and server being v0.5.5

Can we get a WARNING message when running client that points out version mismatch and possible compatibility issues could occur. At least then the cryptic failure would have some context.

```
-- rottaway@stg-ci-agent52:~ $ nomad plan chaos_containers.nomad
Error during plan: Unexpected response code: 500 (rpc error: 2 error(s) occurred:

* Task group membomb validation failed: 1 error(s) occurred:

* 1 error(s) occurred:

* Interval can not be less than 5s (got 0s)
* Task group netbomb validation failed: 1 error(s) occurred:

* 1 error(s) occurred:

* Interval can not be less than 5s (got 0s))
-- rottaway@stg-ci-agent52:~ $ echo $NOMAD_ADDR
http://http.nomad.service.consul:4646
```

### Reproduction steps

With client @ v0.5 and server @ v0.5.5

`nomad plan chaos_containers.nomad`

### Nomad Server logs (if appropriate)

### Nomad Client logs (if appropriate)

### Job file (if appropriate)

chaos_containers.nomad:
```
job "chaos_containers" {
datacenters = ["us-west-2", "us-west-1"]
type = "batch"

periodic {
cron = "*/15 * * * * *"
prohibit_overlap = true
}

group "cpubomb" {
constraint {
attribute = "${meta.pd_hostname}"
value = "stg-clustermgr-client16"
}

task "cpubomb" {
driver = "docker"
config {
image = "monitoringartist/docker-killer"
command = "cpubomb"
}

resources {
cpu = 200
memory = 128
}
}
}

group "membomb" {
count = 1

constraint {
attribute = "${meta.pd_hostname}"
value = "stg-clustermgr-client16"
}

task "membomb" {
driver = "docker"
config {
image = "monitoringartist/docker-killer"
command = "membomb"
args = ["--oom-kill-disable"]
}

resources {
cpu = 20
memory = 128
}
}

restart {
mode = "fail"
}
}

group "netbomb" {
constraint {
attribute = "${meta.pd_hostname}"
set_contains = "stg-clustermgr-client16"
}

task "netbomb" {
driver = "docker"
config {
image = "monitoringartist/docker-killer"
command = "membomb"
}

env {
NETBOMB = "iperf -c 45.33.39.39 -p 5201 -i 1 -u -t 120 -P 4"
}

resources {
cpu = 20
memory = 128
}
}

restart {
mode = "fail"
}
}
}
```

Start at the `nomad plan chaos_containers.nomad` entry point and reproduce the report with client v0.5 and server v0.5.5. Trace where the client and server versions are available, then verify that completion includes a clear mismatch warning and compatibility context without hiding the plan error.

Written by the indexing model from the issue text.