Handle missing `@Reference` gracefully

[elicore]

When using @Reference to hold another document's key name, if the referred document is missing (deleted due to e.g. processing issues, eviction and so on), reading the referring document throws a NPE when the library tries to read the referred document which doesn't exist.

Suggested solution: catch the error in this case and throw a more specific exception (e.g. MissingReferenceError/MissingDocumentError).

Version used: 0.9.11