Expand validation error descriptions

[dgeb]

The description for validation errors will now include all the descriptions of associated validation issues, which may in turn contain descriptions for nested validation issues.

For instance, instead of the generic description record operation is invalid, an expanded description such as the following will be used:

record operation is invalid
- relationship data is invalid
  - relatedRecord has a type 'person' which is not an allowed type for this relationship

Closes #890