PP: Rationalize return values of MacroExpand.

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

Test/baseResults/cppBad2.vert.out

@@ -1,7 +1,6 @@
 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.
+ERROR: 1 compilation errors.  No code generated.
 
 
 Shader version: 100

Test/baseResults/preprocessor.bad_arg.vert.err

@@ -0,0 +1,4 @@
+ERROR: 0:8: 'macro expansion' : End of input in macro EXP2
+ERROR: 1 compilation errors.  No code generated.
+
+

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))), );