D-12007: API Checker Error Message Propogation

Added a natural ordering for Result with Result.compare & the Ordered trait
so that the most salient ErrorResult can be selected from a MultiErrorResult.

Priority is dictated by ErrorResult type & then path length in the automaton.

Updated MultiErrorResult to pass all method calls to the most salient ErrorResult
in its PriorityQueue.  Added allResults method which returns a Traversable of
all Results in the MultiErrorResult.

Added test cases to verify priority dictated by ErrorResult and value of
Result.allResults.  Current tests already verify priority by path length.
Merge pull request #151 from usnavi/D-12007

Issue: 51: D-12007: API Checker Error Message Propogation