Added error output to the preprocessor.

This patch distinguishes preprocessing errors with normal parsing
errors and gives glslangValidator the ability to output preprocessing
errors.

Test/preprocessor.errors.vert

@@ -1,9 +1,9 @@
 #version 310 es
 
-#define X
+#define X 1
 
 #if X
-  #if Y
+  #ifdef Y
     #error This should not show up in pp output.
   #endif
     #error This should show up in pp output.
@@ -11,5 +11,10 @@
   #error This should not show up in pp output.
 #endif
 
+#def X
+#if Y
+
+#extension a
+
 int main() {
 }