refactor!: fetch.Response.Close now returns error (BREAKING CHANGE)

[spiegel-im-spiegel]

Note: This PR contains a breaking change. The signature of fetch.Response.Close has been changed to return an error, so call sites must be updated.

BREAKING CHANGE section:

• Change: the Response Close method now returns an error (previously it did not).
• Impact: code that calls defer resp.Close() or calls resp.Close() directly.
• How to handle: as described in the Migration section above, when calling directly handle the returned error; when using defer, handle the error inside an anonymous function.