check ENOTDIR for readlink

[butonic]

{
  "level": "error",
  "service": "storage",
  "pkg": "rgrpc",
  "traceid": "24d2f1bb7b3cb88c68fd005ced336cce",
  "error": "Decomposedfs: Wrap: readlink error: readlink /var/tmp/ocis/storage/users/nodes/a5066dc8-8566-4aa4-9286-d63c30bdc9a6/somename: not a directory",
  "time": "2021-03-29T19:01:05Z",
  "caller": "/home/vscode/go/pkg/mod/github.com/cs3org/reva@v1.6.1-0.20210326165326-e8a00d9b2368/internal/grpc/services/storageprovider/storageprovider.go:571",
  "message": "error stating: path:\"/home/testfile.txt/somename\" "
}

readlink returns ENOTDIR when a path segment is a file and not a directory. os.IsNotExist won't cover this case: golang/go#18974

part of owncloud/ocis#1239

Links: https://www.man7.org/linux/man-pages/man2/readlink.2.html

[update-docs]

Thanks for opening this pull request! The maintainers of this repository would appreciate it if you would create a changelog item based on your changes.

[butonic]

remaining apiWebdavProperties1/copyFile.feature tests require returning a new error type that we can use to map the AlreadyExists error ...

[butonic]

@ishank011 I also removed the failing test from eos because I don't expect it to be an issue for eos ...

[aduffeck]

LGTM 👍

You could maybe consider adding a unit test to the Describe("Child", ... block in node_test.go for it, e.g.

		It("handles (broken) links including file segments by returning an non-existant node", func() {
			child, err := parent.Child(env.Ctx, "file1/broken")
			Expect(err).ToNot(HaveOccurred())
			Expect(child).ToNot(BeNil())
			Expect(child.Exists).To(BeFalse())
		})