Include the traceback when we fail to extract a name from source

git-svn-id: svn+ssh://src.earth.threerings.net/narya/trunk@6419 542714f4-19e9-0310-aa3c-eee0fc999fb1

src/main/java/com/threerings/presents/tools/GenTask.java

@@ -233,7 +233,7 @@ public abstract class GenTask extends Task
         try {
             name = GenUtil.readClassName(source);
         } catch (Exception e) {
-            throw new BuildException("Failed to parse " + source + ": " + e.getMessage());
+            throw new BuildException("Failed to parse " + source, e);
         }
         return loadClass(name);
     }