all: Add filesystem notification support

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3986

lib/fs/errorfs.go

@@ -6,7 +6,10 @@
 
 package fs
 
-import "time"
+import (
+	"context"
+	"time"
+)
 
 type errorFilesystem struct {
 	err    error
@@ -39,3 +42,6 @@ func (fs *errorFilesystem) Roots() ([]string, error)
 func (fs *errorFilesystem) Usage(name string) (Usage, error)                            { return Usage{}, fs.err }
 func (fs *errorFilesystem) Type() FilesystemType                                        { return fs.fsType }
 func (fs *errorFilesystem) URI() string                                                 { return fs.uri }
+func (fs *errorFilesystem) Watch(path string, ignore Matcher, ctx context.Context, ignorePerms bool) (<-chan Event, error) {
+	return nil, fs.err
+}