Patch image download with invalid _id

[wanliAlex]

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
  bug fix

• What is the current behavior? (You can also link to an open issue here)
  A 500 error is returned if a document with invalid _id is indexed to an image index.

E.g., creating an index with treat_urls_and_pointers_as_image=True and indexing this document
{"_id": 123, "image_field": "https://a-url-to-image/image1.jpg"}
with give you a 500 error.

• What is the new behavior (if this is a feature change)?
  only 400 is returned for the document with an invalid id. The other documents in the batch will be indexed as expected.

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)
  no

• Have unit tests been run against this PR? (Has there also been any additional testing?)
  no

• Related Python client changes (link commit/PR here)
  no

• Related documentation changes (link commit/PR here)
  no

• Other information:
  no

• Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes/features)
• Docs have been added / updated (for bug fixes / features)