registerurl,unregisterurl: rework output and support --json

* registerurl, unregisterurl: Improved output when reading from stdin
  to be more like other batch commands.
* registerurl, unregisterurl: Added --json and --json-error-messages options.

Note that this did change the --batch output in a way that could possibly
break something that expected the old output to never change. I think it's
acceptable to break that because there has never been a guarantee of
unchanging output format except with --batch for most commands. The old
output was just really weird too!

One possible wart is that "git-annex registerurl" with no options now
seems to just hang, since it's waiting for stdin input. Before, it said
"registerurl (stdin)" which was clearer about what's happenening. But this
is a deprecated mode anyway, --batch makes clear what's happening. If
anything, this problem would be a reason to eventually remove the support
for reading from stdin w/o --batch.

Sponsored-by: Dartmouth College's Datalad project

Command/FromKey.hs

@@ -86,12 +86,15 @@ start matcher force (si, (keyname, file)) = do
 -- the uri scheme, to see if it looks like the prefix of a key. This relies
 -- on key backend names never containing a ':'.
 keyOpt :: String -> Key
-keyOpt s = case parseURI s of
+keyOpt = either giveup id . keyOpt'
+
+keyOpt' :: String -> Either String Key
+keyOpt' s = case parseURI s of
 	Just u | not (isKeyPrefix (uriScheme u)) ->
-		Backend.URL.fromUrl s Nothing
+		Right $ Backend.URL.fromUrl s Nothing
 	_ -> case deserializeKey s of
-		Just k -> k
-		Nothing -> giveup $ "bad key/url " ++ s
+		Just k -> Right k
+		Nothing -> Left $ "bad key/url " ++ s
 
 perform :: AddUnlockedMatcher -> Key -> RawFilePath -> CommandPerform
 perform matcher key file = lookupKeyNotHidden file >>= \case