Merge lp://qastaging/~jml/txpkgme/tell-about-cronjob into lp://qastaging/txpkgme

Say where the output file lives and refer to the cronjob when e2e check fails

It's sometimes a little bit confusing when there's an end-to-end check failure,
because the nagios check doesn't actually do the check -- it reads from a cronjob.

Rather than having to remind ourselves of this all the time, I've put a note
in the check failure, pointing to where the file is and mentioning the cronjob.

Failed checks will now look like this::
    Check failed in 12.400000: SOME ERROR. NB: Based on results stored in 'filename' by a cronjob

This changes the interface to check_saved_output to take the filename. It's
not ideal, as its first argument 'output' is the contents of that filename,
but I think it'll do for this.

While here, I changed the main() bit to use 'filename' rather than args[0],
and filled in a missing '%s' in the error when the file doesn't exist.

Thanks,
jml