Improve wallet unlocking error message

[ixje]

Today I was trying to setup a new node and have it automatically open a wallet. I was greeted with a

Failed to open file wallet.json

error message. Reading this message without looking at the source code suggested to me that it couldn't find the physical file, but after debugging I found it was actually failing to unlock the wallet

https://github.com/neo-project/neo-node/blob/a636e8b98acf5df81b967b41bde820b421be429a/neo-cli/CLI/MainService.cs#L395-L410

For a better user experience I'd like to suggest to change the error message to something like

Failed to unlock wallet < filename_and_path >