Nicer error message when no kitten name is specified

__main__.py

@@ -33,7 +33,12 @@ def launch(args):
 
 
 def run_kitten(args):
-    kitten = args[1]
+    try:
+        kitten = args[1]
+    except IndexError:
+        from kittens.runner import list_kittens
+        list_kittens()
+        raise SystemExit(1)
     sys.argv = args[1:]
     from kittens.runner import run_kitten
     run_kitten(kitten)

kittens/runner.py

@@ -101,6 +101,14 @@ def all_kitten_names():
     return ans
 
 
+def list_kittens():
+    print('You must specify the name of a kitten to run')
+    print('Choose from:')
+    print()
+    for kitten in all_kitten_names():
+        print(kitten)
+
+
 def get_kitten_cli_docs(kitten):
     sys.cli_docs = {}
     run_kitten(kitten, run_name='__doc__')