Add subschema titles to ErrorDetails

[lalloni]

Having such details on reported errors allows to use them
on localized error messages/templates for giving better error
context to caller/user.

As an example we're using this like this:

func (l locale) ConditionThen() string {
	return `Must validate "{{ if .thentitle }}{{ .thentitle }}{{ else }}then{{ end }}" as "{{ if .iftitle }}{{ .iftitle }}{{ else }}if{{ end }}" was valid`
}

func (l locale) NumberAllOf() string {
	return `Must validate all subschemas {{ if .subtitles }}{{ range .subtitles }}"{{ . }}"{{ end }}{{ else }}(allOf){{ end }}`
}

// etc...

I didn't include in the PR any modifications to DefaultLocale to preserve default behavior, but I could include that if requested.

[lalloni]

This might help with #214

[johandorland]

First off thank you for your PR!
I'm looking at this with the discussion in #228 in the back of my head regarding errorMessage. If we're considering propagating fields like errorMessage, why not do the same for other fields like $comment, title and description? That'll give schema authors a lot more information on where an error came from.

Your PR takes quite a different approach and from the looks of it. I understand the need for extra information in the ErrorDetails() and using the title of the subschema is a decent way of doing it. The fundamental problem is that there is no API to retrieve nested errors and all the errors are just put in a linear array and the user has to interpret which errors belong together. With that in mind however until we make a new API (the upcoming draft-08 basically requires one with it's new outputting directives) stuffing ErrorDetails() with more information is the way to go.

I have a few points of feedback on your PR:

• If title is not present it will just put in a nil value. If you just put in the title only when it's present users can write more idiomatic code instead of having to write nil checks.
• The updated locale is a nice trick, but I don't think it should be done by default. As "title" is a core keyword it is used a lot in schemas. Users might not expect their errors to change whether a "title" is present or not. Not everyone using "title" in their schema will use it for the specific purpose you are using it for.
• For the sake of consistency: If we add the titles of subschema for *Of and if/then/else, I think it's weird not to have the title of the main schema for all errors.
• In my mind subtitles are part of movies. It might be a language thing, but I'd go for a less ambiguous term.