[jsonschema/schema.go] Improve error handling

[SamuelMarks]

I used the popular errcheck tool to find unhandled errors:

$ errcheck ./...                                                          
jsonschema/resolve.go:448:15:  setAnchor(s, baseInfo, anchorName, false)        
jsonschema/resolve.go:463:13:  setAnchor(s, baseInfo, s.Anchor, false)        
jsonschema/resolve.go:464:13:  setAnchor(s, baseInfo, s.DynamicAnchor, true)

Then I prepared this open-source [Apache-2.0] offline developer tool go-auto-err-handling to automatically add the if err := …; err != nil { return err } lines.

Thanks for reviewing and 🤞 merging