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/model/model.go

@@ -1656,11 +1656,15 @@ func (m *model) GetIgnores(folder string) ([]string, []string, error) {
 		ignores = ignore.New(fs.NewFilesystem(cfg.FilesystemType, cfg.Path))
 	}
 
-	if err := ignores.Load(".stignore"); err != nil && !fs.IsNotExist(err) {
-		return nil, nil, err
+	err := ignores.Load(".stignore")
+	if fs.IsNotExist(err) {
+		// Having no ignores is not an error.
+		return nil, nil, nil
 	}
 
-	return ignores.Lines(), ignores.Patterns(), nil
+	// Return lines and patterns, which may have some meaning even when err
+	// != nil, depending on the specific error.
+	return ignores.Lines(), ignores.Patterns(), err
 }
 
 func (m *model) SetIgnores(folder string, content []string) error {