lfs: improve the hints for common errors in the Batch API The previous message was too debug-ish and less action oriented than a hint should be. The remaining errors that aren't handled are more along the lines of programming errors (not using POST, bad accept type, etc), so I'm not bothering with that. The friendly errors purposely use `self.baseurl` instead of the full Batch API endpoint because I'd expect some copy/paste/modify on the part of the user here, and it would be more confusing if '/objects/batch' magically appeared, but shouldn't be used in the config setting. It still seems like the right thing for debugging in the catchall case.