[2.7] add-cloud and add-credential return types should be more consistent

[Joshua Genet]

When adding a cloud, if it already exists, juju returns ERROR.
When adding a credential, if it already exists, juju returns WARNING.

These should either both be ERRORs or both be WARNINGs.
The output could be a bit more unified as well.

ubuntu@vsphere$ juju add-cloud vsphere cloud.yaml --client
ERROR use `update-cloud vsphere --client` to override known definition: local cloud "vsphere" already exists
ubuntu@vsphere$ echo $?
1

ubuntu@vsphere$ juju add-credential vsphere -f credentials.yaml --client
WARNING credential "steve" for cloud "vsphere" already exists locally, use 'juju update-credential vsphere steve -f credentials.yaml' to update this local client copy
No local credentials for cloud "vsphere" changed.
ubuntu@vsphere$ echo $?
0

ubuntu@vsphere$ juju version
2.7-rc1-bionic-amd64

[Anastasia (anastasia-macmood)]

There are subtle differences between 'add-cloud' and 'add-credential' commands. The most predominant one is that 'add-credential' command supports adding several credentials at once whereas 'add-cloud' can only add one cloud at a time.

For that reason 'add credential' was showing WARNING instead of ERROR to ensure that the processing of potentially other credentials can continue.

It's true that it is surprising to a user since something that was asked did not happen. So, Juju should really err out in this instance.

I have proposed a PR that fixes this and other similar occurrences in cloud/credential commands: https://github.com/juju/juju/pull/10864