When a pristine.hashed file is missing, the only kind of error message
we get is something like
-----8<------
darcs failed: error opening
_darcs/pristine.hashed/e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855
----->8------
Darcs should tell the user that they can run "darcs repair" to attempt
repairing this.