Fixes stockrecord availability validation message in the case when stock is lower than allocation. Previously the message that appeared contained negative numbers. For example: "a maximum of -1 can be bought". Now it will display: "no stock available"

Also make the availability warnings start with a lowercase char so they
can be embedded in other sentences easily.

Rather than a StockRecord instance.  This makes it much simpler.

This are to replace the partner wrappers.