k8s: add default_namespace exception handling

Adds exception handling around the get_default_namespace function, ensuring that if the kubectl
invocation fails, we get some kind of error message especially with instruction to install kubectl
if for some reason the user has not installed it.

src/warnet/k8s.py

@@ -1,5 +1,6 @@
 import json
 import os
+import sys
 import tempfile
 from pathlib import Path
 
@@ -115,7 +116,16 @@ def delete_pod(pod_name: str) -> bool:
 
 def get_default_namespace() -> str:
     command = "kubectl config view --minify -o jsonpath='{..namespace}'"
-    kubectl_namespace = run_command(command)
+    try:
+        kubectl_namespace = run_command(command)
+    except Exception as e:
+        print(e)
+        if str(e).find("command not found"):
+            print(
+                "It looks like kubectl is not installed. Please install it to continue: "
+                "https://kubernetes.io/docs/tasks/tools/"
+            )
+        sys.exit(1)
     return kubectl_namespace if kubectl_namespace else DEFAULT_NAMESPACE