Reply with errors if the expected account does not exist.
Reply with errors if the expected account does not exist.

This fixes the cases for the walletlock and walletpassphase requests
to return an appropiate error to the frontend when the default account
does not exist.

decred/dcrwallet,btcsuite/btcwallet,jrick/dcrwallet,decred/dcrwallet,Stash-Crypto/btcwallet,jrick/btcwallet,btcsuite/btcwallet,jrick/dcrwallet,Stash-Crypto/btcwallet,synrg-labs/btcwallet,synrg-labs/btcwallet,jrick/btcwallet,Roasbeef/btcwallet,Roasbeef/btcwallet