Odd error message when specifying ARM resource ID in role assignment

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**Describe the bug**
When a user specifies an ARM resource ID for a principal as part of a role assignment, the error message returned is not particularly helpful to explain that a resource ID is not a valid input. This may be confusing for a user, given that the target resource is specified as an ARM resource ID in the same command.

Similarly when assigning a user-assigned managed identity to a resource, the ARM resource ID is used, not the object/principal ID.

A more meaningful error message would help reduce confusion.

**To Reproduce**
1. Add role assignment, specifying an ARM resource ID for the assignee - e.g. `az role assignment create --role "Storage Blob Data Reader" --scope /subscriptions//resourceGroups/cliResourceGroup/providers/Microsoft.Storage/storageAccounts/ --assignee /subscriptions//resourceGroups//providers/Microsoft.ManagedIdentity/userAssignedIdentities/`
2. Observe error message

**Expected behavior**
1. The user should get an error message specifying the acceptable inputs that are required. Ideally it would specifically say to use one of these rather than an ARM resource ID.

**Environment summary**
Terminal on OSX
```
az --version
azure-cli 2.25.0

core 2.25.0
telemetry 1.0.6

Extensions:
account 0.2.1
```

**Additional context**