#64 ProductIO.readProduct must throw an IOException

[Hartmut Embacher]

The ProductIO must throw an IOException if
readProduct() was called with an URL where no product
exists.
The firsts step in the Product IO must be to try open
the product file an throw an IO Exception if the
product file can not be opened.
In the second step the product io can evaluate the
product reader plugin to open the produt. If no
matching product reader is available the product io
must return null.