This allows appropriate action to be taken, like displaying
the reason to the user.
Do just that in simplewallet, which should help a lot in
determining why users fail to send.
Also make it so a tx which is accepted but not relayed is
seen as a success rather than a failure.
Block reward may now be less than the full amount allowed.
This was breaking the bitflipping test.
We now keep track of whether a block which was accepted by the core
has a lower than allowed block reward, and allow this in the test.
