JSSpecCompiler: Start converting crashes to error messages

I got fed up with looking at error messages that tell me "VERIFICATION
FAILED: !is_error()". So this commit introduces DiagnosticEngine class
whose purpose is to accumulate and print more user-friendly errors.

Meta/Lagom/Tools/CodeGenerators/JSSpecCompiler/CMakeLists.txt

@@ -17,6 +17,7 @@ set(SOURCES
     Parser/SpecParser.cpp
     Parser/TextParser.cpp
     Parser/XMLUtils.cpp
+    DiagnosticEngine.cpp
     Function.cpp
     main.cpp
 )