az network vpn-connection create - fails in an unclear way if gateway cannot be found

### **This is autogenerated. Please review and update as needed.**

## Describe the bug
When creating a vnet to vnet connection and you don't specify the full ID of the second gateway (or even the first if the connection is in a different RG), then the error you get back is a mysterious "InternalServerError". There could be some validation/checking taking place prior in order to capture the fault and give better feedback to the user. After some trial and error I realised I needed to provide the full ID of the second gateway as it exists in a different resource group, but it would have been helpful to receive a clearer error.

**Command Name**
`az network vpn-connection create`

**Errors:**
`
Deployment failed. Correlation ID: 4bd1c373-004f-48e0-bde6-d6b8a7c79e27. {
"status": "Failed",
"error": {
"code": "ResourceDeploymentFailure",
"message": "The resource operation completed with terminal provisioning state 'Failed'.",
"details": [
{
"code": "InternalServerError",
"message": "An error occurred.",
"details": []
}
]
}
}
`
## To Reproduce:
Steps to reproduce the behavior. Note that argument values have been redacted, as they may contain sensitive information.

- _Put any pre-requisite steps here..._
- `az network vpn-connection create -g {} --name {} --vnet-gateway1 {} --vnet-gateway2 {} --enable-bgp --shared-key {}`

## Expected Behavior

## Environment Summary
```
Linux-4.4.0-18362-Microsoft-x86_64-with-debian-buster-sid
Python 3.6.5
Shell: bash

azure-cli 2.0.63

Extensions:
azure-firewall 0.1.0
front-door 0.1.1

```
## Additional Context