pkg/mount: wrap mount/umount errors

The errors returned from Mount and Unmount functions are raw
syscall.Errno errors (like EPERM or EINVAL), which provides
no context about what has happened and why.

Similar to os.PathError type, introduce mount.Error type
with some context. The error messages will now look like this:

> mount /tmp/mount-tests/source:/tmp/mount-tests/target, flags: 0x1001: operation not permitted

or

> mount tmpfs:/tmp/mount-test-source-516297835: operation not permitted

Before this patch, it was just

> operation not permitted

[v2: add Cause()]
[v3: rename MountError to Error, document Cause()]
[v4: fixes; audited all users]
[v5: make Error type private; changes after @cpuguy83 reviews]

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

pkg/mount/unmount_unix.go

@@ -6,12 +6,17 @@ import "golang.org/x/sys/unix"
 
 func unmount(target string, flags int) error {
 	err := unix.Unmount(target, flags)
-	if err == unix.EINVAL {
+	if err == nil || err == unix.EINVAL {
 		// Ignore "not mounted" error here. Note the same error
 		// can be returned if flags are invalid, so this code
 		// assumes that the flags value is always correct.
-		err = nil
+		return nil
 	}
 
-	return err
+	return &mountError{
+		op:     "umount",
+		target: target,
+		flags:  uintptr(flags),
+		err:    err,
+	}
 }