Pull Request resolves #29325 .

• I read the Generative AI policy and my contribution is either not created with the help of AI or is compatible with the policy and GNU/GPL 2 or later.

Summary of Changes

When something goes wrong during the installation, the installer currently expands all three form
blocks at once (site name, login data, database). It's really hard to find the issue and a lot of scrolling.

This PR makes the installer show only the block the error belongs to:

• Client side validation errors open the block that contains the invalid field and focus it.
• Errors from the server open the database block, which is where the majority of
  them occur (wrong credentials, unknown database, prefix problems, etc.).

The // Reveal everything block in checkInputs() also ran before validation on every click of
"Install Joomla", so even a simple client side error opens the accordion although nothing had
been submitted yet => removed.

Additionally bugs in this context are fixed.

• InstallationController::dbcheck() => $r->error = true: when
  the server side form validation failed the installer stopped the progress bar with nothing visible at all.
• The onError handler in setup.js read response.message, but response does not exist. Now we're using xhr.responseText and output the real message.

Testing Instructions

Apply PR and run npm i. Start a fresh installation (delete/rename configuration.php), fill in the site name and the login data.

1. On the database block enter a wrong database password and press "Install Joomla".
2. Refresh page in browser to restart installation. Enter 1abc-! as the table prefix.
3. on the database block, open the browser dev tools and clear
   #jform_site_name, then press "Install Joomla".

Actual result BEFORE applying this Pull Request

1. All three blocks are open, the error message is at the top and you have to scroll through the whole form to find the database fields again.
2. The installer stays on the progress bar. No block ever comes back, the installation appears to hang and the only way out is a reload.
3. All three blocks are open, and they were opened before validation even ran.

Expected result AFTER applying this Pull Request

1. Only the database block is visible, with the error message above it.
2. Only the database block is visible, with the prefix error message. No more hanging progress bar.
3. Only the site name block is visible and the field is focused and marked invalid.