Add context to Ruleset validation output

[hayfield]

The validator module contains two types of validation: truthy and detailed.

• Truthy validation states merely whether a Dataset is valid against given constraints or not.
• Detailed validation provides information about what errors have occurred and where.

At the moment, detailed Ruleset validation will provide information about which Rule(s) have failed, though nothing about the context. Codelist validation, however, provides additional information such as line numbers so that context can be extracted.

Detailed Ruleset validation should provide context in the output so that it's easier to determine what needs fixing and where.

How this works will vary slightly between different types of Rule (some Rules may need to provide multiple contexts), though it will be possible to obtain useful output for at least some types of Rule.

[hayfield]

Completing #163 first will likely make this a simpler task to complete.