The prior behavior with cloud authentication was a bit confusing
when authenticating against anything but https://pulumi.com/. This
change fixes a few aspects of this:
* Improve error messages to differentiate between "authentication
failed" and "you haven't logged into the target cloud URL."
* Default to the cloud you're currently authenticated with, rather
than unconditionally selecting https://pulumi.com/. This ensures
$ pulumi login -c https://api.moolumi.io
$ pulumi stack ls
works, versus what was currently required
$ pulumi login -c https://api.moolumi.io
$ pulumi stack ls -c https://api.moolumi.io
with confusing error messages if you forgot the second -c.
* To do this, our default cloud logic changes to
1) Prefer the explicit -c if supplied;
2) Otherwise, pick the "currently authenticated" cloud; this is
the last cloud to have been targeted with pulumi login, or
otherwise the single cloud in the list if there is only one;
3) https://pulumi.com/ otherwise.
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