[Compute] add error message for vm identity assign

[xfz11]

Description

resolve #17682
command:
az vm identity assign

result
Before:

'NoneType' object is not callable
Traceback (most recent call last):
python3.8/site-packages/knack/cli.py, ln 215, in invoke
    cmd_![image](https://user-images.githubusercontent.com/81600993/114687941-d0209580-9d46-11eb-8237-6365b365ea73.png) = self.invocation.execute(args)
cli/core/commands/__init__.py, ln 654, in execute
    raise ex
cli/core/commands/__init__.py, ln 718, in _run_jobs_serially
    results.append(self._run_job(expanded_arg, cmd_copy))
cli/core/commands/__init__.py, ln 711, in _run_job
    six.reraise(*sys.exc_info())
...
cli/command_modules/vm/custom.py, ln 648, in assign_vm_identity
    assign_identity_helper(cmd.cli_ctx, getter, setter, identity_role=identity_role_id, identity_scope=identity_scope)
cli/core/commands/arm.py, ln 1057, in assign_identity
    resource = setter(resource)
cli/command_modules/vm/custom.py, ln 642, in setter
    vm.identity.user_assigned_identities[identity] = VirtualMachineIdentityUserAssignedIdentitiesValue()
TypeError: 'NoneType' object is not callable

After

History Notes

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