vfs: add tests and subsequent fixes

* Tests for VFS layer
  * Small fixes found during testing
  * Fix Close, Flush and Release behaviour for ReadFileHandle and WriteFileHandle
  * Fix nil object bugs on File

vfs/errors_test.go

@@ -0,0 +1,13 @@
+package vfs
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestErrorError(t *testing.T) {
+	assert.Equal(t, "Success", OK.Error())
+	assert.Equal(t, "Function not implemented", ENOSYS.Error())
+	assert.Equal(t, "Low level error 99", Error(99).Error())
+}