Revert change that removed default traceback in debug builds.

README.html

@@ -1014,7 +1014,6 @@ and at <a href="http://luaj.sourceforge.net/api/2.0/index.html">http://luaj.sour
 <li>Make shared string metatable an actual metatable.</li>
 <li>Add sample code that illustrates techniques in creating sandboxed environments.</li>
 <li>Add convenience methods to Global to load string scripts with custom environment.</li>
-<li>Turn off default traceback.  Use xpcall() with debug.traceback instead.</li>
 <li>Move online docs to <a href="http://luaj.org/luaj/3.0/api/index.html">http://luaj.org/luaj/3.0/api/</a></li>
 
 </ul></td></tr>

src/core/org/luaj/vm2/LuaClosure.java

@@ -522,7 +522,9 @@ public class LuaClosure extends LuaFunction {
 		if (globals == null ) return msg;
 		final LuaThread r = globals.running;
 		if (r.errorfunc == null)
-			return msg;
+			return globals.debuglib != null?
+					msg + "\n" + globals.debuglib.traceback(level):
+					msg;
 		final LuaValue e = r.errorfunc;
 		r.errorfunc = null;
 		try {