Final minor fixes to getter & setter, new "MissingAttr" exception.

In particular fixed bug where obligatory getter would not throw error when attribute or entity was missing.
Introduced new exception "MissingAttr" to be thrown when an obligatory getter does not find required attribute.
Fixed some setter not throwing messages, in particular fixed "link" setter that would remove attribute falsly.
Merge pull request #158 from balint42/fix_obligatory_getter

Fix obligatory getter