Fix Old Sign Conversion

1) Sign loading code was trying to parse the JSON before the check for oldSign.
       That code could then skip the old sign converting code if it triggers a JSON parse exception.
    2) New Mojang Schematic system has Tile Entities in the new converted format, but missing the Bukkit.isConverted flag
       This causes Igloos and such to render broken signs. We fix this by ignoring sign conversion for Defined Structures

scripts/importmcdev.sh

@@ -53,6 +53,7 @@ import CommandScoreboard
 import CommandWhitelist
 import DataBits
 import DataConverterMaterialId
+import DefinedStructure
 import EULA
 import EntitySquid
 import EntityWaterAnimal