compose-tracker: support finding task id for failed failable tasks
    
    If a single arch of an image build fails (i.e. the image build fails
    for ppc64le, but succeeds for others) and that arch is failable then
    we'd never get the task ID in the opened issue because the task ID
    wasn't on the next line in the output. With this patch we parse through
    until we find the task ID and then return that.