Allow to get source position of unmarshal error

[rhysd]

Fixes #759

With this PR, error instances in TypeError.Errors know their source positions. Previously the field was string slice, but now it is slice of struct of message, line and column so that the caller can know where each error occurred exactly.

[andig]

It would be absolutely fantastic to have this capability.

/cc @niemeyer is this blocked in any way? Could a small sponsoring help to move this forward?

[andig]

@rhysd I think there is a second case that's not handled. When decode panics, it moves into handleErr:

func handleErr(err *error) {
	if v := recover(); v != nil {
		if e, ok := v.(yamlError); ok {
			*err = e.err
		} else {
			panic(v)
		}
	}
}

Does yamlError need line information, too?

[rhysd]

It would be necessary. But I don't have enough time to update this branch for now.

[andig]

@rhysd Maybe you could take a look- forked your PR and added terminal failures with line numbers.

[rhysd]

Looks good to me. Thank you for improving this branch. I'm closing this in favor of #901.