Better error handling in treasure disks

- Return a more sensible string for empty treasure disks (i.e. those
   given by /give). This should help identify packs which are giving
   items in non-supported ways.
 - Fix NPE when the treasure mount doesn't exist.

Fixes #801

src/main/java/dan200/computercraft/shared/media/items/ItemTreasureDisk.java

@@ -68,6 +68,8 @@ public String getLabel( @Nonnull ItemStack stack )
     public IMount createDataMount( @Nonnull ItemStack stack, @Nonnull World world )
     {
         IMount rootTreasure = getTreasureMount();
+        if( rootTreasure == null ) return null;
+
         String subPath = getSubPath( stack );
         try
         {
@@ -121,7 +123,7 @@ private static IMount getTreasureMount()
     private static String getTitle( @Nonnull ItemStack stack )
     {
         CompoundNBT nbt = stack.getTag();
-        return nbt != null && nbt.contains( NBT_TITLE ) ? nbt.getString( NBT_TITLE ) : "'alongtimeago' by dan200";
+        return nbt != null && nbt.contains( NBT_TITLE ) ? nbt.getString( NBT_TITLE ) : "'missingno' by how did you get this anyway?";
     }
 
     @Nonnull