Fixed incorrect verifier code for nested tables.

It was outputting the type instead of the field name, and didn't deal
with NULL fields. Added test case.

Also fixed token enums having the wrong value, resulting in
unreadable error messages.

Change-Id: Icd9b4d22f417bfad5824c0f58e067ce3f2e2dc6f
Tested: on Windows and Linux.

tests/test.cpp

@@ -91,7 +91,7 @@ std::string CreateFlatBufferTest() {
   // shortcut for creating monster with all fields set:
   auto mloc = CreateMonster(builder, &vec, 150, 80, name, inventory, Color_Blue,
                             Any_Monster, mloc2.Union(), // Store a union.
-                            testv, vecofstrings, vecoftables);
+                            testv, vecofstrings, vecoftables, 0);
 
   builder.Finish(mloc);