add missing CHECKURL-FAILURE ErrorMsg to docs

doc/design/external_special_remote_protocol.mdwn

@@ -226,7 +226,7 @@ the special remote can reply with `UNSUPPORTED-REQUEST`.
     one after the other.
     Note that since a list is returned, neither the Url nor the Filename
     can contain spaces.
-  * `CHECKURL-FAILURE`  
+  * `CHECKURL-FAILURE ErrorMsg`  
     Indicates that the requested url could not be accessed.
 * `WHEREIS Key`  
   Asks the remote to provide additional information about ways to access

doc/todo/external_remotes__58___Add_support_for_ErrorMsg_to_-FAILURE_messages_lacking_it.mdwn

@@ -28,3 +28,5 @@ git-annex version: 8.20200810+git143-gdee38c54d-1~ndall+1
 
 [[!meta author=yoh]]
 [[!tag projects/datalad]]
+
+> [[fixed|done]] the documentation --[[Joey]]

doc/todo/external_remotes__58___Add_support_for_ErrorMsg_to_-FAILURE_messages_lacking_it/comment_1_a096df37d5bc5543b574b32554311af9._comment

@@ -0,0 +1,19 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 1"""
+ date="2020-09-18T15:50:11Z"
+ content="""
+CHECKURL-FAILURE actually supports an error message,
+always has. The docs were just wrong.
+
+CHECKPRESENT-FAILURE is not for an exceptional case,
+but a usual case. CHECKPRESENT-UNKNOWN is for the exceptional case and
+includes an error message.
+
+CLAIMURL-FAILURE means the remote does not claim the url, so there is no
+need for an error message since git-annex will use web as usual.
+
+WHEREIS-FAILURE means the remote does not have information for whereis,
+it's not an exceptional case. Especially since WHEREIS is not supposed to
+use network access or anything.
+"""]]