pkg/lockfile: wrap error in openLock

unix.Open returns a bare error (like EPERM). This is worked around in
on user, (*lockfile).lock, where we panic, but not in the other user,
createLockerForPath, and as a result such a bare error might be returned
from GetLockfile or GetROLockfile.

Wrap it into os.PathError and remove the workaround from lockfile.lock.

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

pkg/lockfile/lockfile_unix.go

@@ -89,7 +89,7 @@ func openLock(path string, ro bool) (fd int, err error) {
 		return openLock(path, ro)
 	}
 
-	return
+	return fd, &os.PathError{Op: "open", Path: path, Err: err}
 }
 
 // createLockerForPath returns a Locker object, possibly (depending on the platform)
@@ -156,7 +156,7 @@ func (l *lockfile) lock(lType int16, recursive bool) {
 		// If we're the first reference on the lock, we need to open the file again.
 		fd, err := openLock(l.file, l.ro)
 		if err != nil {
-			panic(fmt.Sprintf("error opening %q: %v", l.file, err))
+			panic(err)
 		}
 		l.fd = uintptr(fd)