Value quantity, serialize test observations, improve exceptions

[aeyates]

This pull request tackles three areas:

• Add conversion logic for observations with a ValueQuantity and ReferenceRange
• Serialize JSON resources into observations for tests.
• Categorize exceptions for more streamlined method signatures.

[kirenotneb]

@aeyates and I had a brief side-bar about the way exceptions are being handled here. We'll likely look at future refinements to either create a hierarchy of exceptions, a la MalformedLoincException extends LoincException or look at returning richer metadata about what transpired while attempting to process the request.