add some extra error information

contracts/eosio.token/eosio.token.hpp

@@ -72,14 +72,14 @@ namespace eosio {
    asset token::get_supply( symbol_name sym )const
    {
       stats statstable( _self, sym );
-      const auto& st = statstable.get( sym );
+      const auto& st = statstable.get( sym , "get_supply : can not find symbol");
       return st.supply;
    }
 
    asset token::get_balance( account_name owner, symbol_name sym )const
    {
       accounts accountstable( _self, owner );
-      const auto& ac = accountstable.get( sym );
+      const auto& ac = accountstable.get( sym , "get_balance : can not find asset symbol");
       return ac.balance;
    }