Help with writer needs asset target path

I took some time to understand that AssetWriter breaks if assets are not given explicitly a targetPath. I thought Assetic would deduce that from either AssetManager's name or Asset's source. As it is, trying by copy&paste from documentation's example will lead to errors and I'm not the first one being caught by this (http://stackoverflow.com/questions/14538335/assetic-unable-to-write-assets).

So I made it return a meaningful error and updated documentation.

Thank you
Federico