Improve error handling

Currently when imagemagick display warnings like this:

```
identify: iCCP: known incorrect sRGB profile `letter.png' @ warning/png.c/MagickPNGWarningHandler/1832.
PNG 8 1024 1024 20634B 28.350000000000001421 letter.png
```
node-easyimage takes this as fatal error and return immediately with a `File not supported` error.

I've updated error handling to manage the case where warnings are returned in `stderr` alongs with response in `stdout`. I've also passed `stderr` as Error message rather than unhelpful `File not supported` error.