Fix error with fs.DirExists()

It errors out when providing a subdirectory of a file, e.g. `index.md/.zk`

internal/adapter/fs/fs.go

@@ -79,11 +79,7 @@ func (fs *FileStorage) FileExists(path string) (bool, error) {
 
 func (fs *FileStorage) DirExists(path string) (bool, error) {
 	fi, err := fs.fileInfo(path)
-	if err != nil {
-		return false, err
-	} else {
-		return fi != nil && (*fi).Mode().IsDir(), nil
-	}
+	return !os.IsNotExist(err) && fi != nil && (*fi).Mode().IsDir(), nil
 }
 
 func (fs *FileStorage) fileInfo(path string) (*os.FileInfo, error) {

internal/core/notebook_store.go

@@ -46,7 +46,7 @@ func (e ErrNotebookNotFound) Error() string {
 // Open returns a new Notebook instance for the notebook containing the
 // given file path.
 func (ns *NotebookStore) Open(path string) (*Notebook, error) {
-	wrap := errors.Wrapper("open failed")
+	wrap := errors.Wrapper("failed to open notebook")
 
 	path = ns.fs.Canonical(path)
 	nb := ns.cachedNotebookAt(path)