Nodes with solo migrations appear to be getting spurious errors when rebuilding their wallet

[jshufro]

Seems like rocketpool wallet rebuild does not filter custom-keys before attempting to recover, and fails on solo migrations.

Perhaps this is because the custom-keys directory is empty when rocketpool minipool import-key runs.

Fix alongside #407 - instead of returning a nondescript error, inform the user which keys were recovered and which could not be. The user should be able to determine whether or not further action is required from this.