[enh] Add details to errors

[senpro-ingwersenk]

Hello!

So, I just spent a good four hours trying to figure out why I could no longer deserialize my data into a struct and I kept getting this error here:

go-json/internal/errors/error.go

Lines 159 to 164 in 9872089

	func ErrInvalidBeginningOfValue(c byte, cursor int64) *SyntaxError {
	return &SyntaxError{
	msg: fmt.Sprintf("invalid character '%c' looking for beginning of value", c),
	Offset: cursor,
	}
	}

But actually... the error was because the field I was trying to serialize into was "an object" rather than "a string". Once I changed that (which I did because of a very random lightbulb-turns-on moment) the deserialization worked again exactly as I would have expected it to.

Finding this was ... well, let's just say my day is a little ruined. x)

Therefore I would love to propose adding more context into errors:

• Segment that was attempted to be parsed (a range, perhaps)
• What the value was ment to be serialized into (i.e. ...while unmarshaling %s into %T or something)
• Probably a reminder for dumb people like me that unmarshaling is dictated by the target type (very optional though.... cough :) )

Having had a little more information on the error could've saved me a lot of troubble today - so, after my little realization, I thought I might as well put this out here as a feature request.

Thank you and kind regards!