Improve error message when users don't configure the password for password-protected SSH keys

[pjbgf]

Flux supports the use of password-protected SSH keys. When they are being used, users must provide it via a password field on the same secret in which the identity (private key) and known_hosts are provided.

If users forget to provide such a password, they end up getting the error below: error: error creating SSH agent: "SSH agent requested but SSH_AUTH_SOCK not-specified"}

The error message could be improved to make it easier for users to realise what the problem is.

xref: https://fluxcd.io/docs/components/source/gitrepositories/#ssh-authentication