Show more meaningful JSON errors

[kzantow]

What would you like to be added:
When attempting to decode Syft JSON (or other formats), error messages do not show what line/character an error occurred, making it very difficult to understand what's wrong or how to fix it.

Why is this needed:
A user is unable to proceed when unable to parse a Syft JSON file.

Additional context:
It looks like there is a fairly simple way to get more information about parser errors when decoding JSON. See this article.

The gist is, the error returned from json.Unmarshal needs to be tested for types: *json.SyntaxError and *json.UnmarshalTypeError. Both of these types provide a .Offset field which is the character offset where an error occurred. From there we can write a simple function to extract the line text, line number, and character number to provide a nicely formatted message about the error, something like:

invalid JSON input line 256 (expected number):
>     "field": "value",
>              ^

The original function outlined in the article is, which could easily be modified for our needs:

func lineAndCharacter(input string, offset int) (line int, character int, err error) {
	lf := rune(0x0A)

	if offset > len(input) || offset < 0 {
		return 0, 0, fmt.Errorf("Couldn't find offset %d within the input.", offset)
	}

	// Humans tend to count from 1.
	line = 1

	for i, b := range input {
		if b == lf {
			line++
			character = 0
		}
		character++
		if i == offset {
			break
		}
	}

	return line, character, nil
}

This is related to #1351

[wagoodman]

Are there any other JSON libs that can help further identify these kinds of errors?