BUG: Errors while listing zones do not propagate through the system (#2474)

[hmoffatt]

If an error occurred listing zones (eg invalid credentials), the preview/push commands didn't return an error code.

[hmoffatt]

Also discovered that CLOUDFLAREAPI caches an empty zone list if the fetch fails.

[tlimoncelli]

Good catch!

CC @tresni for approval.

[hmoffatt]

I notice that the CLOUDNS code fetches the domain list in NewCloudns in order to validate the credentials, while NewCloudflare does not. That validation runs earlier so Cloudns already exits nicely with an error if the credentials are wrong, while CLOUDFLARE ends up trying all the domains with the same error.

[tlimoncelli]

Providers shouldn't generate any network traffic just at initialization time. I guess I missed that when I reviewed ClouDNS.

We should probably add a .CheckCreds() method or something. But so far nobody has complained not getting an error until the first domain of that provider.

[hmoffatt]

I've pushed another change for CLOUDNS to delay fetching the domain list.

[tlimoncelli]

Thanks for fixing this!!!