[drone] improve export error and progress output

[jimsheldon]

When running harness-migrate drone export, if the token provided does not have access to a repository in the requested namespace, only a Not Found error is returned.

We should improve the output around every step that the drone export subcommand runs.

For example, currently if a repository in the namespace is active, but has no execution history, it is skipped. We should tell the user that the repository is being skipped and why.