Incorrect error reported by New-AzureRmAutomationWebHook

If I attempt to create a webhook for a runbook and I specify an invalid runbook name

``` ps1
New-AzureRmAutomationWebHook -ResourceGroupName 'valid-group' -AutomationAccountName 'valid-account' -Name 'webhook1' -RunbookName 'invalid-runbook' -IsEnabled $true -ExpiryTime ((Get-Date).AddYears(1)) -Force
```

I get the following error:

```
New-AzureRmAutomationWebHook : The Automation account was not found.
```

The automation account does exist, but the runbook does not, so the error is misleading.