Subnet create, unhelping error message if overlapping address range

## Describe the bug

**Issue:**
Non helping error message

**Command Name**
`az network vnet subnet create`

**Errors:**
```
Subnet 'private-dmz' is not valid in virtual network 'dmz'.
```

## To Reproduce:
The only requirement to reproduce this issue is to create a subnet with an overlapping address range on another subnet (set with the --address-prefixes option) .

### Steps explanation
2) In the following example, we have a 64 address range available in our 'dmz' vnet.
3) We create a first 16 address-ranged subnet (public-dmz). at 10.0.0.0/28 ~[10.0.0.0 - 10.0.0.15]
4) We create a second 32 address-ranged subnet (private-dmz) at 10.0.0.0/27 ~[10.0.0.0 - 10.0.0.32].

The 4th step is the command leading to the imprecise error message.

As you can see, the range of the second subnet 'private-dmz' overlap the first one's range.

### Steps to reproduce the behavior:

1) `az group create -n myGroup`
2) `az network vnet create -g myGroup -n dmz --address-prefixes 10.0.0.0/26`
3) `az network vnet subnet create -g myGroup --vnet-name dmz -n public-dmz --address-prefixes 10.0.0.0/28`
4) `az network vnet subnet create -g myGroup --vnet-name dmz -n private-dmz --address-prefixes 10.0.0.0/27`

Independently of the issue, the correct 4th step would be:
`az network vnet subnet create -g home --vnet-name dmz -n private-dmz --address-prefixes 10.0.0.32/27`

## Expected Behavior
The user is the one who is getting wrong but the occurring error message isn't helping at all in leading the user to resolve the mistake he did.

It would be interesting to append an error message like the following one:
```
New subnet's address range would overlap already existing 'public-dmz' subnet.
````

## Environment Summary
```
Linux-4.15.0-1060-azure-x86_64-with-debian-stretch-sid
Python 3.6.5
Shell: bash

azure-cli 2.0.75

Extensions:
interactive 0.4.3

```