Improve error handling around compact-cluster installs

Worker machinesets generated, during create manifests can be deleted
before creating the cluster. Detect this case and take action based
on whether masters are marked as schedulable.
This fixes compact cluster installs on GCP, AWS and Azure.

Author: sadasu

pkg/asset/cluster/tfvars/tfvars.go

@@ -272,6 +272,13 @@ func (t *TerraformVariables) Generate(parents asset.Parents) error {
 		if err != nil {
 			return err
 		}
+		// Based on the number of workers, we could have the following outcomes:
+		// 1. workers > 0, masters not schedulable, valid cluster
+		// 2. workers = 0, masters schedulable, valid compact cluster but currently unsupported
+		// 3. workers = 0, masters not schedulable, invalid cluster
+		if len(workers) == 0 {
+			return errors.Errorf("compact clusters with 0 workers are not supported at this time")
+		}
 		workerConfigs := make([]*machinev1beta1.AWSMachineProviderConfig, len(workers))
 		for i, m := range workers {
 			workerConfigs[i] = m.Spec.Template.Spec.ProviderSpec.Value.Object.(*machinev1beta1.AWSMachineProviderConfig) //nolint:errcheck // legacy, pre-linter
@@ -370,6 +377,13 @@ func (t *TerraformVariables) Generate(parents asset.Parents) error {
 		if err != nil {
 			return err
 		}
+		// Based on the number of workers, we could have the following outcomes:
+		// 1. workers > 0, masters not schedulable, valid cluster
+		// 2. workers = 0, masters schedulable, valid compact cluster but currently unsupported
+		// 3. workers = 0, masters not schedulable, invalid cluster
+		if len(workers) == 0 {
+			return errors.Errorf("compact clusters with 0 workers are not supported at this time")
+		}
 		workerConfigs := make([]*machinev1beta1.AzureMachineProviderSpec, len(workers))
 		for i, w := range workers {
 			workerConfigs[i] = w.Spec.Template.Spec.ProviderSpec.Value.Object.(*machinev1beta1.AzureMachineProviderSpec) //nolint:errcheck // legacy, pre-linter
@@ -485,6 +499,13 @@ func (t *TerraformVariables) Generate(parents asset.Parents) error {
 		if err != nil {
 			return err
 		}
+		// Based on the number of workers, we could have the following outcomes:
+		// 1. workers > 0, masters not schedulable, valid cluster
+		// 2. workers = 0, masters schedulable, valid compact cluster but currently unsupported
+		// 3. workers = 0, masters not schedulable, invalid cluster
+		if len(workers) == 0 {
+			return errors.Errorf("compact clusters with 0 workers are not supported at this time")
+		}
 		workerConfigs := make([]*machinev1beta1.GCPMachineProviderSpec, len(workers))
 		for i, w := range workers {
 			workerConfigs[i] = w.Spec.Template.Spec.ProviderSpec.Value.Object.(*machinev1beta1.GCPMachineProviderSpec) //nolint:errcheck // legacy, pre-linter