[Bug]: Enhance error message when missing policy defaults

### Which product(s) are you raising for?

ALZ Terraform (AVM)

### What versions of the tools are you using?

- ALZ AVM module version: 0.19.0
-

### What is the bug?

We mistakenly specified the amba defaults as module inputs, but forgot to include the library reference in the alz provider.

The plan fails and the alz provider reports a random amba default as missing (every run another random amba default is reported).

Used code (you should include this in your issue template):

```terraform
provider "alz" {
library_overwrite_enabled = true
library_references = [
{
path = "platform/alz",
ref = "2025.02.0"
},
# mistakenly left out:
# {
# path = "platform/amba",
# ref = "2025.07.0"
# },
{
custom_url = "${path.root}/lib"
}
]
}

module "avm-ptn-alz" {
source = "Azure/avm-ptn-alz/azurerm"
version = "~> 0.0"
location = "westeurope"
architecture_name = "alz"
parent_resource_id = data.azapi_client_config.current.tenant_id

subscription_placement = local.subscription_placement

policy_assignments_to_modify = local.policy_assignments_to_modify

policy_default_values = {
# ALZ
log_analytics_workspace_id = jsonencode({ value = local.log_analytics_workspace_id })
ama_change_tracking_data_collection_rule_id = jsonencode({ value = local.ama_change_tracking_data_collection_rule_id })
ama_user_assigned_managed_identity_id = jsonencode({ value = local.ama_user_assigned_managed_identity_id })
ama_user_assigned_managed_identity_name = jsonencode({ value = local.ama_user_assigned_managed_identity_name })
ama_vm_insights_data_collection_rule_id = jsonencode({ value = local.ama_vm_insights_data_collection_rule_id })

# AMBA
# amba_alz_action_group_email = jsonencode({ value = local.amba_alz_action_group_email })
amba_alz_management_subscription_id = jsonencode({ value = "${var.management_subscription_id}" })
amba_alz_resource_group_location = jsonencode({ value = local.amba_alz_resource_group_location })
amba_alz_resource_group_name = jsonencode({ value = local.amba_alz_resource_group_name })
amba_alz_user_assigned_managed_identity_name = jsonencode({ value = local.amba_alz_user_assigned_managed_identity_name })
amba_alz_byo_user_assigned_managed_identity_id = jsonencode({ value = local.amba_alz_byo_user_assigned_managed_identity_id })
}
}
```

### Log Output

```Text
╷
│ Error: architectureDataSource.Read() Error applying policy assignment default `amba_alz_byo_user_assigned_managed_identity_id`
│
│ with module.avm-ptn-alz.data.alz_architecture.this,
│ on .terraform\modules\avm-ptn-alz\main.tf line 1, in data "alz_architecture" "this":
│ 1: data "alz_architecture" "this" {
│
│ Hierarchy.AddDefaultPolicyAssignmentValue: A default with name
│ `amba_alz_byo_user_assigned_managed_identity_id` does not exist
╵
```

### Screenshot(s)

_No response_