remote-helper: better error when key is not in ssh-agent
The following happened during a user test:
- He then created a new commit and tried to run git push rad but that failed due to the missing ssh-agent.
- I had him add RAD_PASSPHRASE= git push rad which did the trick, we should probably provide an interactive prompt for people to enter their passphrase?
Without hand holding, this may be confusing to a new user. We should explicitly tell them about the use of ssh-agent and point them towards being able to use RAD_PASSPHRASE if ssh-agent is not an option.
This 100% hung me up for a long minute. If nothing else, maybe mention that you need to set the env var in the guide.