Front-end: Fix known crashes by early exit on error (issue #29, issue #34, issue #35).

Added -C option to request cascading errors.  By default, will exit early,
to avoid all error-recovery-based crashes.

This works by simulating end-of-file in input on first error, so no
need for exception handling, or stack unwinding, or any complex error
checking/handling to get out of the stack.

gtests/Initializer.h

@@ -51,7 +51,7 @@ namespace glslangtest {
 // gets fixed.
 class GlslangInitializer {
 public:
-    GlslangInitializer() : lastMessages(EShMsgDefault)
+    GlslangInitializer() : lastMessages(EShMsgCascadingErrors)
     {
         glslang::InitializeProcess();
     }