Show more details on errors for New-AzVirtualNetwork when there are duplicate Subnets

## Problem
When using array object for the Subnet parameter of New-AzVirtualNetwork command. If there is a duplicate subnet (same name) we get the following error:
```
StatusCode: 400
ReasonPhrase: Bad Request
OperationID : 'f5cdb7f6-c616-40fd-91b3-c20d05bba58c'
At line:1 char:9
+ $VNet = New-AzVirtualNetwork -Name $VNetName -ResourceGroupName $RGNa ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : CloseError: (:) [New-AzVirtualNetwork], NetworkCloudException
+ FullyQualifiedErrorId : Microsoft.Azure.Commands.Network.NewAzureVirtualNetworkCommand
```
By enabling -Debug we can see the actual issue:
```
"error": {
"code": "InvalidRequestFormat",
"message": "Cannot parse the request.",
"details": [
{
"code": "DuplicateResourceName",
"message": "Resource /subscriptions//resourceGroups//providers/Microsoft.Network/virtualNetworks/ has two child resources with the same name (SL-App)."
}
]
}

```

## Proposed implementation details (optional)
Allow the error message to include why the command has failed instead of juts saying 400 / Bad request