message: duplicated error message
  There is a duplicated error message for database_exception and guard_exception. Maybe if the error were exposed to users like developer, they may be confused about what really happens without checking the exact error code.
diff: @@ -230,7 +230,7 @@ namespace eosio { namespace chain {
  3060004, "Contract Query Exception" )
  FC_DECLARE_DERIVED_EXCEPTION( guard_exception, database_exception,
  - 3060100, "Database exception" )
  + 3060100, "Guard Exception" )
  FC_DECLARE_DERIVED_EXCEPTION( database_guard_exception, guard_exception,
  3060101, "Database usage is at unsafe levels" )
  