Group validation errors by oneOf subschema

[Bjwebb]

In OpenDataServices/cove#895 we now assume whether a record has linked or embedded releases, in order to use the correct subschema within the oneOf block. Text about this assumption is added to every relevant validation message.

This text is repeated for each validation error message. Instead we should group the messages by subschema used, and state the assumption only once.

[jpmckinney]

Possible related: #126

[duncandewhurst]

Unlike #126, which only affects records with a mix of embedded and linked release, this issue affects any record with issues in its releases array.

[duncandewhurst]

Here's an example of some data that triggers the addition of the text to several errors: https://gist.github.com/duncandewhurst/b15299d96b97304c87cb16a8b201c3e6

I find the way it is reported currently quite confusing when the validation message is about an object other than the release, e.g.