add an error message if passphrase is not associated with any wallet

[pbca26]

the error is needed in case there's a typo or simply wrong passphrase was used expected behaviour is to see an error

right now iguana just creates a new wallet with passphrase provided

[jl777]

changed things so walletpassphrase returns error unless encryptwallet was used previously to create the walletfile.

any assumption that walletpassphrase autocreates the wallet and autologs in will now be broken