- Change is_XXX to return std::error_condition instead of bool so that errors can be passed down the line; rename these to check_is_XXX to indicate that they are no longer basic predicates
- Change return type of internal function metadata_find to std::error_condition so that errors can be returned rather than thrown
* imagedev/cdromimg.cpp: Fix bug where cdrom_file object could be constructed twice in a row for CD-ROMs and once for DVDs
