Various

- Overhaul Failure types to concrete CoreDataRepositoryError
- Cleanup various internal code for less possible error branches and readability
- Fix lots of wrong doc comments
- Fix sporadic test failures

Author: roanutil

.swiftpm/xcode/xcshareddata/xcschemes/CoreDataRepository.xcscheme

@@ -74,7 +74,9 @@
       </EnvironmentVariables>
       <Testables>
          <TestableReference
-            skipped = "NO">
+            skipped = "NO"
+            parallelizable = "YES"
+            testExecutionOrdering = "random">
             <BuildableReference
                BuildableIdentifier = "primary"
                BlueprintIdentifier = "CoreDataRepositoryTests"