fix panic when connection to controller fails

[SimoneDutto]

Description

This pr fixes a bug that causes Jimm to panic when it is issued juju status on a not-existing model.

I think this bug is even bigger because Jimm will crash on a failing controller's connection generally.

How to reproduce the bug

• set a qa environment with make qa-microk8s
• remove from the database the model (ex. make dev-env-cleanup and make dev-env again)
• juju status

Jimm will panic. It can't find the model in the database, so it is not able to create the connection. Therefore, it will later try to read Json from a nil connection.

This fix returns the error and adds a test for it.

Before the fix

and juju status hangs

After the fix

juju status will return ERROR model not found (not found)

Fixes JIRA/GitHub issue number

Engineering checklist

Check only items that apply

• Documentation updated
• Covered by unit tests
• Covered by integration tests

Test instructions

Notes for code reviewers

[alesstimec]

good find, thanks!