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/test-preprocessor-list

@@ -5,3 +5,4 @@ preprocessor.function_macro.vert
 preprocessor.line.vert
 preprocessor.pragma.vert
 preprocessor.simple.vert
+preprocessor.success_if_parse_would_fail.vert