#13 Add errorproperty attribute to all tasks

[Karl]

If an errorproperty attribute can be added to all top level tasks (rapc, sigtool etc) then it would be easier to determine if the tasks have passed or failed. This is especially important with sigtool where even failing to receive valid signature responses doesn't not ultimately force the containing build to fail.

In most cases I would imagine this optional attributes value would simply be passed straight through to the underlying Java/Exec calls.