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.
With the change in mixin rules for v2, the "annoying" outputs are
slightly changed. There is high correlation between dust and
unmixable, but no equivalence.
This obsoletes the need for a lengthy blockchain rescan when
a transaction doesn't end up in the chain after being accepted
by the daemon, or any other reason why the wallet's idea of
spent and unspent outputs gets out of sync from the blockchain's.
