gather: Collect and re-raise errors from gather method
    
    When there is an exception in gathering (such as after seeing unsigned
    packages in deps method), the exception was lost and the compose
    continued to run until it tried to access the result and crashed on
    KeyError.
    
    Relates: https://pagure.io/releng/failed-composes/issue/587
    JIRA: COMPOSE-3986
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>