json object for FileNotFound

When a nonexistant file is passed to a command and  --json-error-messages
is enabled, output a JSON object indicating the problem.

(But git ls-files --error-unmatch still displays errors about such files in
some situations.)

I don't like the duplication of the name of the command introduced by this,
but I can't see a great way around it. One way would be to pass the Command
instead.

When json is not enabled, the stderr is unchanged. This is necessary
because some commands like find have custom output. So dislaying
"find foo not found" would be wrong. So had to complicate things with
toplevelFileProblem having different output with and without json.

When not using --json-error-messages but still using --json, it displays
the error to stderr, but does display a json object without the error. It
does have an errorid though. Unsure how useful that behavior is.

Sponsored-by: Dartmouth College's Datalad project

Messages/JSON.hs

@@ -20,6 +20,7 @@ module Messages.JSON (
 	addErrorMessage,
 	note,
 	info,
+	errorid,
 	add,
 	complete,
 	progress,
@@ -51,6 +52,7 @@ import Utility.Metered
 import Utility.Percentage
 import Utility.Aeson
 import Utility.FileSystemEncoding
+import Types.Messages
 
 -- A global lock to avoid concurrent threads emitting json at the same time.
 {-# NOINLINE emitLock #-}
@@ -68,7 +70,8 @@ emit' b = do
 	putMVar emitLock ()
 
 -- Building up a JSON object can be done by first using start,
--- then add and note any number of times, and finally complete.
+-- then add and note and errorid any number of times, and finally
+-- complete.
 type JSONBuilder = Maybe (Object, Bool) -> Maybe (Object, Bool)
 
 none :: JSONBuilder
@@ -112,6 +115,12 @@ note s (Just (o, e)) = Just (HM.unionWith combinelines (HM.singleton "note" (toJ
 		String (old <> "\n" <> new)
 	combinelines new _old = new
 
+errorid :: ErrorId -> JSONBuilder
+errorid _ Nothing = Nothing
+errorid eid (Just (o, e)) = Just (HM.unionWith replaceold (HM.singleton "errorid" (toJSON' (show eid))) o, e)
+  where
+	replaceold new _old = new
+
 info :: String -> JSONBuilder
 info s _ = case j of
 	Object o -> Just (o, True)