PP: Rationalize return values of MacroExpand.

This results in better error recovery, including fewer
crashes on badly formed PP input.

Test/preprocessor.bad_arg.vert

@@ -0,0 +1,8 @@
+#define M(a) a
+int M(aou
+    = 2)  // Okay, one argument, split across newline
+    ;
+
+// end of file during an argument
+#define EXP2(a, b)
+EXP2(((((1,2,3,4))), );