Ticket #48914 - db2bak.pl task enters infinitive loop when bak fs is almost full
    
    Description: A backend helper function dblayer_copyfile returns an error
    when any of the copy operation fails.  But one of the caller functions
    dblayer_backup ignored the error.
    
    This patch checks the error returned from dblayer_copyfile and abort the
    back-up.
    
    Also, more error info is added to the log messages.
    
    https://fedorahosted.org/389/ticket/48914
    
    Reviewed by mreynolds@redhat.com (Thank you, Mark!!)
    
    (cherry picked from commit ff997cd6fa5f2a0678721ba0b6a56fdce327feb0)