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/Link.FromFile.cpp

@@ -48,8 +48,7 @@ TEST_P(LinkTest, FromFile)
 {
     const auto& fileNames = GetParam();
     const size_t fileCount = fileNames.size();
-    const EShMessages controls = DeriveOptions(
-            Source::GLSL, Semantics::OpenGL, Target::AST);
+    const EShMessages controls = DeriveOptions(Source::GLSL, Semantics::OpenGL, Target::AST);
     GlslangResult result;
 
     // Compile each input shader file.