Fix update indexes running when not necessary

[silvanocerza]

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• The PR has no duplicates (please search among the Pull Requests
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• The PR follows
  our contributing guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

• What kind of change does this PR introduce?

An overall enhancement of the CLI behaviour.

• What is the current behavior?

If a local index file is missing the CLI tries to download it from the related additional URL whenever it tries to create a new internal instance, if that URL is unreachable for any reason the subsequent command always fails.
This would happen when executing any command.

• What is the new behavior?

Indexes update is now only executed for certain specific commands like update, core update-index, lib update-index and core search, or whenever the library_index.json or package_index.json files are missing.

The initialization of an internal CLI instance still makes the whole command fail if a local index file is missing but the message is clearer now.

Updating indexes now shows a clear error if an URL is unreachable for any reason, we check for 4xx or 5xx HTTP status codes so in those cases it fails.

• Does this PR introduce a breaking change?

No.

• Other information:

None.

This should also fix #1050.

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[silvanocerza]

Updated the PR with a different behaviour, now we make the CLI fail if local indexes related to a certain URL are not found, in these cases the user should try to update the indexes.
We also enhanced the error messages when updating indexes with update and core update-index if any URL is not reachable.