[OTA] Cancel installation when mounting /system failed, and enable installation on devices with systemless root

This merge requests addresses two problems that can be encountered during OTA .zip installation.

Cancel installation if /system couldn't be mounted (commit ab7befb3)

Previous behavior

When mounting of /system fails, the installation continues and "succeeds" (no error message). When you reboot to Android, nothing is installed.

New behavior

When mounting /system fails, an error message is displayed and the installation is cancelled.

Add OTA .zip compatibility with "systemless root" devices (mainly devices that use Magisk) (commit 8be1367f)

Similar issue discussed here: https://github.com/d4rken-org/sdmaid/issues/2110

Previous behavior

Installation fails on some "systemless root" devices.

New behavior

Installation succeeds on these devices (only tested with my personal device though).

Side notes

• All new code in this merge request is only executed if the "mount" command fails on /system mount point, so these changes cannot break installations that previously worked (no regression).
• I don't know why the pipeline fails... I'm not familiar with the project, and the issue does not seem to be related to my modifications.