Cryptic error message if Juju uses the wrong json file from GCE

[Jorge Castro]

    $juju bootstrap jorge google
    ERROR json: cannot unmarshal object into Go value of type string

This is because I used the wrong json file as provided by google. Google gives you many options for how to download "cloud credentials". We don't tell the user exactly what json file we mean, so "select auth-type [jsonfile*, oauth2]:" is too vague.

Juju specifically needs a "Service account client" json file, any other one will result in the error above, so we need to figure out a way to let the user know:

a) Which json file.
b) How to get to the URL in google cloud to get the exact json file, which is harder than you'd think.
c) CLI should inform the user of this file
d) If CLI detects a json type that is not "service_account", error out with an appropriate message.

[Tim McNamara (tim-clicks)]

I've recommended a change[0] to our add-credential help documentation that should make it less likely that users will get confused:

[0] https://github.com/juju/juju/pull/10216