DRAFT: expose s3HTTP error checking without parse_response to e.g allow e.g 404's to be surfaced

[slodge]

I'm looking to assist with S3-SELECT support.

To do this I've made a repo https://github.com/slodge/aws.s3.select which has header generating and response parsing methods.

However, these methods need code which is currently buried inside parse_response handling under s3HTTP.

This PR tries to open up that error handling... I'm open to other ways to do this.

Current state

• [x] code changes or improvements - linked to #282
• [x] trivial change - so not adding to DESCRIPTION
• [ ] not yet added to NEWS.md - waiting to hear if this is wanted/welcome - or if alternative suggested
• [ ] edited docs - but not yet rerun the document() - expecting changes...
• [ ] not added any new tests yet
• [ ] R CMD check does not run - have some tests and some confusing docs to work through yet