gfs2_utils: Improve error messages
    
    Currently gfs2_edit complains that it doesn't know what 'master' means
    when it can't find the master directory. This patch catches the case
    where the master directory hasn't been found and reports the error more
    clearly to the user. It also updates a terse 'out of space' error
    message in blk_alloc_i to be more clear.
    
    rhbz#742293
    
    Signed-off-by: Andrew Price <anprice@redhat.com>