Catch parsing errors in image API

Fixes https://sentry.io/organizations/coreos/issues/664440779/

endpoints/api/image.py

@@ -8,11 +8,18 @@ from endpoints.exception import NotFound
 
 
 def image_dict(image, with_history=False, with_tags=False):
+  parsed_command = None
+  if image.command:
+    try:
+      parsed_command = json.loads(image.command)
+    except (ValueError, TypeError):
+      parsed_command = {'error': 'Could not parse command'}
+
   image_data = {
     'id': image.docker_image_id,
     'created': format_date(image.created),
     'comment': image.comment,
-    'command': json.loads(image.command) if image.command else None,
+    'command': parsed_command,
     'size': image.image_size,
     'uploading': image.uploading,
     'sort_index': len(image.parents),