Enhance error handling when querying remote server for image updates

[Tbaile]

Currently the Update view lacks error handling if the remote server is not reachable. This can happen if firewall loses connectivity or if the upstream server are in maintenance mode or so on.

Proposed solution

Changing the API, adding python3-requests helps to gather info on what happened to the request and returns the insights to the user on what went wrong.

PRs:

• UI: fix(updates): gracefully handle error for updates nethsecurity-ui#466
• API: fix(ns-api): gracefully handle error when checking for updates #990

[gsanchietti]

Testing image.

Test case 1

• Install a machine without a subscription
• Verify the machine can access the repository

Test case 2

• Register a machine with a subscription
• Verify the machine can access the repository

Test case 3

• Put distfeed.nethesis.it on maintenance mode
• Verify the UI notifies the user that a maintenance is in progress

[gsanchietti]

Test case 1 and 2 verified.

Test case 3 failed.
This how the page looks like when the remote repository is not accessible:

Command output:

# /usr/libexec/rpcd/ns.update call check-system-update
{"error": "maintenance"}