feat(occ): Better handling of disabled or unreacheable App store

[solracsf]

Summary

Set either appstoreenabled or has_internet_connection to false on config.php file.

Previously in all cases:

# occ app:update previewgenerator
previewgenerator is up-to-date or no updates could be found

After:

# occ app:update previewgenerator
App store access is disabled

or

# occ app:update previewgenerator
Internet connection is disabled, and therefore the default public App store is not reachable

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[provokateurin]

Can you add two tests for this?