Move disk into its own package, remove bloated code

pkg/storage/donut/disk/errors.go

@@ -0,0 +1,17 @@
+package disk
+
+// InvalidArgument invalid argument
+type InvalidArgument struct{}
+
+func (e InvalidArgument) Error() string {
+	return "Invalid argument"
+}
+
+// UnsupportedFilesystem unsupported filesystem type
+type UnsupportedFilesystem struct {
+	Type string
+}
+
+func (e UnsupportedFilesystem) Error() string {
+	return "Unsupported filesystem: " + e.Type
+}