lib/fs, lib/model: Add error channel to Watch to avoid panics (fixes #5697) (#5734)

* lib/fs, lib/model: Add error channel to Watch to avoid panics (fixes #5697)

* forgot unsupported watch

* and more non(-standard)-unixy fixes

* and windows test

* review

lib/fs/basicfs.go

@@ -8,6 +8,7 @@ package fs
 
 import (
 	"errors"
+	"fmt"
 	"os"
 	"path/filepath"
 	"runtime"
@@ -334,3 +335,13 @@ func longFilenameSupport(path string) string {
 	}
 	return path
 }
+
+type ErrWatchEventOutsideRoot struct{ msg string }
+
+func (e *ErrWatchEventOutsideRoot) Error() string {
+	return e.msg
+}
+
+func (f *BasicFilesystem) newErrWatchEventOutsideRoot(absPath, root string) *ErrWatchEventOutsideRoot {
+	return &ErrWatchEventOutsideRoot{fmt.Sprintf("Watching for changes encountered an event outside of the filesystem root: f.root==%v, root==%v, path==%v. This should never happen, please report this message to forum.syncthing.net.", f.root, root, absPath)}
+}