[[ Ext ]] Ensure load if library deletes stacks on failure

The `revInternal__LoadIfLibrary` handler depends on `send` throwing
an error if the handler is not found, however, a recent change meant
that the `revAvailableHanlders` was used to check if the stack had
library initialization handlers so `send` was sending empty which does
not throw an error. See Bug 20689

Author: montegoulding

Toolset/libraries/revinitialisationlibrary.livecodescript

@@ -65,6 +65,8 @@ command revInternal__LoadIfLibrary pLibrary
          // TODO: Remove legacy message name	            
       else if tAvailable contains "revLoadLibrary" then 
          put "revLoadLibrary" into tMsg
+      else
+         throw "not a library"
       end if
       send tMsg to stack pLibrary
       local tStackName