Move standardize_image inside try/except.
Move standardize_image inside try/except.

Inside `make_pool`, we put the I/O operation inside try/except to avoid
letting a couple bad files kill the long operation. This PR moves the
"standardization" step in there too, as I'm seeing errors like

```
Can not convert from object to float64.
```

due to some PNGs not well handled by ``imread``.

danielballan/photomosaic