gui, lib/ignore: Handle editing ignores with error (fixes #5425) (#6757)

This changes the error handling in loading ignores slightly:

- There is a new ParseError type that is returned as the error
  (somewhere in the chain) when the problem was not an I/O error loading
  the file, but some issue with the contents.

- If the file was read successfully but not parsed successfully we still
  return the lines read (in addition to nil patterns and a ParseError).

- In the API, if the error IsParseError then we return a successful
  HTTP response with the lines and the actual error included in the JSON
  object.

- In the GUI, as long as the HTTP call to load the ignores was
  successful we can edit the ignores. If there was an error we show this
  as a validation error on the dialog.

Also some cleanup on the Javascript side as it for some reason used
jQuery instead of Angular for this editor...

lib/api/api.go

@@ -43,6 +43,7 @@ import (
 	"github.com/syncthing/syncthing/lib/discover"
 	"github.com/syncthing/syncthing/lib/events"
 	"github.com/syncthing/syncthing/lib/fs"
+	"github.com/syncthing/syncthing/lib/ignore"
 	"github.com/syncthing/syncthing/lib/locations"
 	"github.com/syncthing/syncthing/lib/logger"
 	"github.com/syncthing/syncthing/lib/model"
@@ -1161,15 +1162,16 @@ func (s *service) getDBIgnores(w http.ResponseWriter, r *http.Request) {
 
 	folder := qs.Get("folder")
 
-	ignores, patterns, err := s.model.GetIgnores(folder)
-	if err != nil {
+	lines, patterns, err := s.model.GetIgnores(folder)
+	if err != nil && !ignore.IsParseError(err) {
 		http.Error(w, err.Error(), 500)
 		return
 	}
 
-	sendJSON(w, map[string][]string{
-		"ignore":   ignores,
+	sendJSON(w, map[string]interface{}{
+		"ignore":   lines,
 		"expanded": patterns,
+		"error":    errorString(err),
 	})
 }
 
@@ -1750,3 +1752,11 @@ func checkExpiry(cert tls.Certificate) error {
 
 	return nil
 }
+
+func errorString(err error) *string {
+	if err != nil {
+		msg := err.Error()
+		return &msg
+	}
+	return nil
+}