Please be kind enough to explain what is missing, what you expect to get and how this should be tested.
Also, you are using hard text in this patch which is untranslatable. Change to JText

Hi @infograf768,
I thought it was enough clear what code was missing, by the way....
scenario:

• extension installation process using the "Upload & Install Joomla Extension"
• installation is failed due to something unexpected in the extension installer but not explained by the Jinstaller.

The only message returned is "Installation unexpectedly terminated:" (check line 397 of ROOT\libraries\cms\installer\installer.php). Why? because the function abort($msg = null, $type = null) is called with an empty error message due to the missing code (check the lines I've modified).
I know the JText should be used, and it also need to be used on line 397 of ROOT\libraries\cms\installer\installer.php, My commit was a quick fix, this is the first time I contribute to the JOOMLA development so I didn't wanna do too many modifications on code.
I'll add the full patch with the language string later......after dinner :-)

@bygiro
When modifying administrator en-GB.lib.joomla.ini, please also update the site/language/en-GB/en-GB.lib.joomla.ini.

Please update your PR to latest staging and fix the merge conflicts.

what "PR" stands for?

Pullrequest. This thing that you are looking at right now.

@bygiro Can you redo this pull request or shall I find someone else to do it? Thanks.

I have tested this item unsuccessfully on f7db062

Patch could not e loaded

Error

The patch could not be applied because the repository is missing

As we can't merge this PR (deleted repo) and the current installer code throws exections instend use abort i'm going to close here. Thanks.