File: explicit the expected storage type (R/W)

A tiny fix used to make the error message a bit clearer, e.g:

```Lua
require 'torch'

local t = torch.rand(3):float()
local f = torch.DiskFile('out.bin', 'w'):binary()
-- This misuse (passing a float storage) gives a `number, or Storage expected`
-- error. It could be easier to debug (IMHO) if the expected type is explicit:
-- `number, or DoubleStorage expected`
f:writeDouble(t:storage())
f:close()
```