Provide detail on why CHECKURL failed for datalad and archive special…

… remotes

Inspired by

- datalad/datalad-container#267

but to be of benefit, requires review/merge/release of

- Lykos153/AnnexRemote#106

But should be safe otherwise since ATM (without the above PR) AnnexRemote simply ignores the message

datalad/customremotes/archives.py

@@ -312,7 +312,7 @@ def checkurl(self, url):
         else:
             # TODO: theoretically we should first check if key is available
             # from any remote to know if file is available
-            return False
+            raise RemoteError(f"archive key {akey} is not available locally.")
 
     def checkpresent(self, key):
         # TODO: so we need to maintain mapping from urls to keys.  Then

datalad/customremotes/datalad.py

@@ -13,17 +13,16 @@
 import logging
 from urllib.parse import urlparse
 
+from datalad.customremotes import RemoteError
+from datalad.customremotes.base import AnnexCustomRemote
+from datalad.customremotes.main import main as super_main
 from datalad.downloaders.providers import Providers
 from datalad.support.exceptions import (
     CapturedException,
     TargetFileAbsent,
 )
 from datalad.utils import unique
 
-from datalad.customremotes import RemoteError
-from datalad.customremotes.base import AnnexCustomRemote
-from datalad.customremotes.main import main as super_main
-
 lgr = logging.getLogger('datalad.customremotes.datalad')
 
 
@@ -72,7 +71,7 @@ def checkurl(self, url):
         except Exception as exc:
             ce = CapturedException(exc)
             self.message("Failed to check url %s: %s" % (url, ce))
-            return False
+            raise RemoteError(str(ce))
 
     def checkpresent(self, key):
         resp = None