Fix json parsing bug

[sivukhin]

I ran govanish linter (it still in the WIP phase) against gval repo and it found that one error handling branch were eliminated by compiler:

2023/12/24 21:05:32 it seems like your code vanished from compiled binary: func=[parseJSONObject], file=[/home/sivukhin/code/gval/parse.go], line=[320], snippet:
	return nil, p.Expected("object", ':')

if p.Scan() != ':' {
	if err != nil {
		return nil, p.Expected("object", ':')
	}
}

Indeed, errcheck in this case is useless and actually prevent parsing from correct behaviour: rejecting invalid JSON structure.

This PR remove err check and always return error if p.Scan() return incorrect symbol.

Also, test Invalid json key:value structure for incorrect JSON structure were added (it were red before fix and became green after)