display error when an invalid url is downloaded

download is documented as displaying an error when download fails, but
it didn't when the url was not valid at all. That leads to confusing
behavior.

Also, display the url with --debug

Utility/Url.hs

@@ -311,7 +311,10 @@ download meterupdate url file uo =
 				(DownloadWithCurl _, _)
 					| isfileurl u -> downloadfile u
 					| otherwise -> downloadcurl
-		Nothing -> return False
+		Nothing -> do
+			liftIO $ debugM "url" url
+			hPutStrLn stderr "download failed: invalid url"
+			return False
 	
 	isfileurl u = uriScheme u == "file:"
 

doc/bugs/unclear_why_annex_fails_to_get_a_file_from_a_public_S3_bucket_where_it_exported_before.mdwn

@@ -120,3 +120,6 @@ It would be nice, before introducing versioning information as you outlined in t
 
 
 [[!meta author=yoh]]
+
+> Added an error message when download of an invalid url is attempted
+> and also made the url be listed by --debug. [[done]] --[[Joey]]

doc/bugs/unclear_why_annex_fails_to_get_a_file_from_a_public_S3_bucket_where_it_exported_before/comment_1_08514c489b916b2ae8e4e0122051856f._comment

@@ -0,0 +1,18 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 1"""
+ date="2018-09-25T17:31:36Z"
+ content="""
+Misconfiguration:
+
+	joey@darkstar:~/tmp/ds001419>git show git-annex:remote.log | grep openneuro.org
+	336309c0-d7f8-430e-bd2b-4bdc61e587cb bucket=openneuro.org datacenter=US encryption=none exporttree=yes fileprefix=ds001419/ host=s3.amazonaws.com name=s3-PUBLIC partsize=1GiB port=80 public=yes publicurl=yes storageclass=STANDARD type=S3 timestamp=1537895418.565736858s
+
+publicurl=yes means that the bucket's url is "yes".
+
+Since constructing an URI object from that invalid url fails, the url
+downloader fails early and this avoids the debug output that would normally be
+shown too when downloading an url by recent versions of git-annex.
+
+This also explains probably why whereis doesn't list any urls.
+"""]]