Improve CAPZ Bootstrapping Extension's error message

Author: willie-yao

azure/defaults.go

@@ -112,7 +112,7 @@ const (
 
 var (
 	// LinuxBootstrapExtensionCommand is the command the VM bootstrap extension will execute to verify Linux nodes bootstrap completes successfully.
-	LinuxBootstrapExtensionCommand = fmt.Sprintf("for i in $(seq 1 %d); do test -f %s && break; if [ $i -eq %d ]; then exit 1; else sleep %d; fi; done", bootstrapExtensionRetries, bootstrapSentinelFile, bootstrapExtensionRetries, bootstrapExtensionSleep)
+	LinuxBootstrapExtensionCommand = fmt.Sprintf("for i in $(seq 1 %d); do test -f %s && break; if [ $i -eq %d ]; then echo 'Error joining node to cluster: kubeadm init failed. To debug, check the cloud-init, kubelet, or other bootstrap logs: https://capz.sigs.k8s.io/self-managed/troubleshooting.html?highlight=kubeadmcontrolplane#checking-cloud-init-logs-ubuntu.'; exit 1; else sleep %d; fi; done", bootstrapExtensionRetries, bootstrapSentinelFile, bootstrapExtensionRetries, bootstrapExtensionSleep)
 	// WindowsBootstrapExtensionCommand is the command the VM bootstrap extension will execute to verify Windows nodes bootstrap completes successfully.
 	WindowsBootstrapExtensionCommand = fmt.Sprintf("powershell.exe -Command \"for ($i = 0; $i -lt %d; $i++) {if (Test-Path '%s') {exit 0} else {Start-Sleep -Seconds %d}} exit -2\"",
 		bootstrapExtensionRetries, bootstrapSentinelFile, bootstrapExtensionSleep)