Don't cancel subtask when other fails
    
    In some cases it makes sense to continue with sibling task even if some of
    them already failed. E.g. with kernel build it could be of use if submitter
    knows for which archs it succeed and for which it fails. Repeated builds
    could take a lot of time and resources.
    
    Option is turned on by 'build_arch_can_fail' in kojid.conf and it is turned
    on per builder. Note, that it shouldn't be enabled by default as it could
    result in unnecessary resource consumption.