Bad Error Message: 'NoneType' object has no attribute when --resource-group or --name is misspelled

**Describe the bug**
`az webapp identity show --resource-group "mispelledgroup" --name "somename"` with a bad resource group or name field and returns a `'NoneType' object has no attribute 'identity'` error. In general it seems that when using `az webapp` if there is anything wrong with the `--resource-group` or `--name` flags it will return a `'NoneType' object has no attribute` error message.

**To Reproduce**
Run `az webapp identity show` with bad --resource-group or --name fields.

**Expected behavior**
A clear and actionable error message.

**Environment summary**
MSI, azure-cli (2.0.54), windows 10

Reproduce `az webapp identity show` with misspelled `--resource-group` and `--name` values, then trace the command's error handling. Done means the command reports a clear, actionable error instead of a `'NoneType' object has no attribute 'identity'` message.

Written by the indexing model from the issue text.