PP: Fix issue #426, recover from bad-source macro expansion.

Test/baseResults/cppBad2.vert.out

@@ -0,0 +1,19 @@
+cppBad2.vert
+ERROR: 0:3: 'macro expansion' : End of input in macro b
+ERROR: 0:3: '' : compilation terminated 
+ERROR: 2 compilation errors.  No code generated.
+
+
+Shader version: 100
+ERROR: node is still EOpNull!
+0:?   Linker Objects
+
+
+Linked vertex stage:
+
+ERROR: Linking vertex stage: Missing entry point: Each stage requires one entry point
+
+Shader version: 100
+ERROR: node is still EOpNull!
+0:?   Linker Objects
+

Test/cppBad2.vert

@@ -0,0 +1,3 @@
+#define a b(
+#define b(x)
+b(a)