[RPC] importprivkey account re-assigning even though it returns an error

[whphhg]

Using importprivkey with an already imported private key, but different account name will re-assign that address to the new account, even though it was previously imported and you're getting an error response.

[whphhg]

It now no longer returns an error when you try to import an existing private key.

Edit: going over the docs I found out that the default is to return null if the private key is successfully imported or already part of the wallet. It was a neat distinction to show better error to the user.