Update error messages for invalid world folder following the format of

Author: benwoo1110

src/main/resources/multiverse-core_en.properties

@@ -273,7 +273,7 @@ mv-core.cloneworld.invalidworldname=World '{world}' contains invalid characters!
 mv-core.cloneworld.worldexistfolder=World '{world}' exists in server folders! You need to delete it first before cloning.
 mv-core.cloneworld.worldexistunloaded=World '{world}' already exists and it's unloaded! You need to delete it first before cloning.
 mv-core.cloneworld.worldexistloaded=World '{world}' already exists! You need to delete it first before cloning.
-mv-core.cloneworld.fromworldfolderinvalid=Source world '{world}' folder contents does not seem to be a valid world! Make sure contains at least a 'level.dat' file.
+mv-core.cloneworld.fromworldfolderinvalid=Source world '{world}' folder contents does not seem to be a valid world! Make sure it contains the correct world structure of data and region folders.
 mv-core.cloneworld.copyfailed=Failed to copy world to '{world}': {error}\n&fSee console for more details.
 
 mv-core.createworld.invalidworldname=World '{world}' contains invalid characters!
@@ -289,7 +289,7 @@ mv-core.deleteworld.failedtodeletefolder=Failed to delete world folder '{world}'
 mv-core.importworld.invalidworldname=World '{world}' contains invalid characters!
 mv-core.importworld.worldexistunloaded=World '{world}' already exists, but it's not loaded!&f Type '&a/mv load {world}&f' if you wish to load it.
 mv-core.importworld.worldexistloaded=World '{world}' already exists!
-mv-core.importworld.worldfolderinvalid=World '{world}' folder contents does not seem to be a valid world! Make sure contains at least a 'level.dat' file.\n&cIf the server software does something different with world folders, or you are very certain the world is valid, use '&3--skip-folder-check&c' flag.
+mv-core.importworld.worldfolderinvalid=World '{world}' folder contents does not seem to be a valid world! Make sure it contains the correct world structure of data and region folders.\n&cIf the server software does something different with world folders, or you are very certain the world is valid, use '&3--skip-folder-check&c' flag.
 mv-core.importworld.bukkitenvironmentmismatch=Environment mismatch detected!&f The world '{world}' is already loaded with environment '{bukkitEnvironment}', but Multiverse is trying to import it with environment '{mvEnvironment}'.
 
 mv-core.loadworld.worldalreadyloading=World '{world}' is already loading! Please wait...