MkdirAll: fix usage

(below is a quote from my runc commit 6f82d4b)

TL;DR: check for IsExist(err) after a failed MkdirAll() is both
redundant and wrong -- so two reasons to remove it.

Quoting MkdirAll documentation:

> MkdirAll creates a directory named path, along with any necessary
> parents, and returns nil, or else returns an error. If path
> is already a directory, MkdirAll does nothing and returns nil.

This means two things:

1. If a directory to be created already exists, no error is
returned.

2. If the error returned is IsExist (EEXIST), it means there exists
a non-directory with the same name as MkdirAll need to use for
directory. Example: we want to MkdirAll("a/b"), but file "a"
(or "a/b") already exists, so MkdirAll fails.

The above is a theory, based on quoted documentation and my UNIX
knowledge.

3. In practice, though, current MkdirAll implementation [1] returns
ENOTDIR in most of cases described in #2, with the exception when
there is a race between MkdirAll and someone else creating the
last component of MkdirAll argument as a file. In this very case
MkdirAll() will indeed return EEXIST.

Because of #1, IsExist check after MkdirAll is not needed.

Because of #2 and #3, ignoring IsExist error is just plain wrong,
as directory we require is not created. It's cleaner to report
the error now.

Note this error is all over the tree, I guess due to copy-paste,
or trying to follow the same usage pattern as for Mkdir(),
or some not quite correct examples on the Internet.

[1] https://github.com/golang/go/blob/f9ed2f75/src/os/path.go

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

content/local/store.go

@@ -62,7 +62,7 @@ func NewStore(root string) (content.Store, error) {
 // require labels and should use `NewStore`. `NewLabeledStore` is primarily
 // useful for tests or standalone implementations.
 func NewLabeledStore(root string, ls LabelStore) (content.Store, error) {
-	if err := os.MkdirAll(filepath.Join(root, "ingest"), 0777); err != nil && !os.IsExist(err) {
+	if err := os.MkdirAll(filepath.Join(root, "ingest"), 0777); err != nil {
 		return nil, err
 	}