[5.4] Expose finalization errors in autoupdate API response

[SniperSister]

Summary of Changes

So far, the update finalization endpoint of the autoupdate feature has not been exposing any potential cause of a issue during that execution step. This PR adds output.

Testing Instructions

• Install a site
• Enable auto updates
• Apply the patch
• Call the finalizeUpdate endpoint of the API /api/index.php/v1/joomlaupdate/finalizeUpdate and don't forget to supply the X-JUpdate-Token header with the respective token
• Verify the existence of the errors key in the response attributes

Actual result BEFORE applying this Pull Request

Errors missing

Expected result AFTER applying this Pull Request

Errors present

Link to documentations

Please select:

• Documentation link for docs.joomla.org:

• No documentation changes for docs.joomla.org needed

• Pull Request link for manual.joomla.org:

• No documentation changes for manual.joomla.org needed