distgit: fix infinite import loop for thunderbird
    
    See #1700
    
    For some reason, `git checkout BRANCH` can fail. In case of
    thunderbird, it fails with
    
        stderr: 'error: Your local changes to the following files would be overwritten by checkout:
    	    get-calendar-langpacks.sh
    	    node-stdout-nonblocking-wrapper
        Please commit your changes or stash them before you switch branches.
    
    I assume it is a packaging issue but we shouldn't try to import this
    package again and again but rather mark it as failed and log the error.