$> grep -e -FAILURE doc/design/external_special_remote_protocol.mdwn
  * `PREPARE-FAILURE ErrorMsg`  
  * `REMOVE-FAILURE Key ErrorMsg`  

so some commands have a way to report why action failed and some do not.

I have been running into an error message CHECKURL failed with no reason given and I thought our special remote is just not letting know details but apparently I just followed the spec and thus did not provide description of the reason why CHECKURL has failed. That complicates debugging/fixing.

Unfortunately it seems that adding such support would not be backward compatible, ie older versions of git annex would puke if external remote starts to provide error message, so I guess it would be up to the remote to check annex version (or protocol features?) on either it can report an error:

  external special remote protocol error, unexpectedly received "CLAIMURL-FAILURE IT IS FAILURE" (unable to parse command)
git-annex: unable to use special remote due to protocol error

$> git annex version
git-annex version: 8.20200810+git143-gdee38c54d-1~ndall+1

fixed the documentation --Joey