Fix incorrect error handling.
Fix incorrect error handling.

This fixes a bug in udb.Store.InsertMinedTx where an error writing the
new mined balance to the database would result in returning early
without indicating any error at all.

jrick/dcrwallet,decred/dcrwallet,jrick/dcrwallet,decred/dcrwallet,jrick/btcwallet,jrick/btcwallet