[Azure-CLI] Better error messages for az functionapp create

Non unique function app name:
```
Website with given name already exists.
```

Perhaps change it to `Website or function app with the given name is already taken.`

Invalid function app name:
```
The host name 20200630-test-.azurewebsites.net is invalid.
```

Perhaps change it to `Function app name is invalid.` (and print out the rules)

Internal work item: https://dev.azure.com/azuretracking/Azure%20tracking/_workitems/edit/1369/