Export error variables that are returned from exported methods; fix d…

[slessard]

Export error variables that are returned from exported methods
fix documentation for Get method

[fatih]

Hi @slessard

If we expose these variables, they will become part of the library. I'm not sure I want to extend the API surface. How do you use it? I'm not sure exporting these variables is useful, but maybe I'm missing something. Thank you

[slessard]

Exporting the error variables would allow me to check the type of error in a manner that is friendly to recent versions of Go using the errors.Is function. Here's an example:

for ;; {
	tags, err := structtag.Parse(string(field.Tag))
	if errors.Is(err, structtag.ErrTagKeySyntax) ||
		errors.Is(err, structtag.ErrTagSyntax) ||
		errors.Is(err, structtag.ErrTagValueSyntax) {
		// These minor errors I can live with
		continue
	} else {
		// Houston, we have a problem.
		return err
	}
	authorizedRolesAsCSVString, err = tags.Get(rolesTagKey)
	if errors.Is(err, structtag.ErrTagNotExist) {
		// This kind of error I can handle. There just aren't any roles
		continue
	} else {
		// Getting tags failed in some unexpected way. Time to bail out
		return err
	}
	// Process tags here
}

Without exported error variables I don't know of a good way to test the error return. This forces me to make assumptions about the structtags functions and the errors they return.

[BrianLeishman]

I see the biggest pro argument as being able to tell whether or not a tag exists, just like @slessard has written an example for. It would be nice to skip an operation if the tag simply doesn't exist

[zzjin]

Any update here for 2 years wait?