cmd-buildextend-live: ensure files exist before writing kargs.json

Rather than silently deleting files that no longer exist, flip this
around so that we assert they all exist. This will then ensure that new
code moving `grub.cfg` will also have to update `kargs_json`.

src/cmd-buildextend-live

@@ -625,10 +625,11 @@ boot
                        '-efi-boot', 'images/efiboot.img',
                        '-no-emul-boot']
 
-    # We've done everything that might affect kargs, so filter out any files
-    # that no longer exist and write out the kargs JSON if it lists any files
-    kargs_json['files'] = [f for f in kargs_json['files']
-            if os.path.exists(os.path.join(tmpisoroot, f['path']))]
+    # Sanity-check that all kargs files that we found earlier still exist. This
+    # ensures that any modifications made also updated kargs_json as needed.
+    for f in kargs_json['files']:
+        fn = os.path.join(tmpisoroot, f['path'])
+        assert os.path.exists(fn), f"{fn} no longer exists"
     kargs_json['files'].sort(key=lambda f: f['path'])
     if kargs_json['files']:
         # Store the location of "karg embed areas" for use by