Improve error message to figure out which fragment class is problematic.

Bug: 8057867
Change-Id: I6ad0bc8c8a6ed9bc8a1cca5debffe15c6de3a1b5

src/com/android/settings/SettingsPreferenceFragment.java

@@ -233,7 +233,10 @@ public class SettingsPreferenceFragment extends PreferenceFragment implements Di
                     mParentFragment = getFragmentManager().findFragmentById(mParentFragmentId);
                     if (!(mParentFragment instanceof DialogCreatable)) {
                         throw new IllegalArgumentException(
-                                KEY_PARENT_FRAGMENT_ID + " must implement "
+                                (mParentFragment != null 
+                                        ? mParentFragment.getClass().getName()
+                                        : mParentFragmentId)
+                                + " must implement "
                                 + DialogCreatable.class.getName());
                     }
                 }