As of 0.9990,#1910 is still not completely fixed. A failed install (such as failure to install the driver) now exits with a proper exit code, but still leaves files behind, most importantly %SystemRoot%\System32\Npcap\wpcap.dll and the HKLM\Software\Npcap registry branch. This is causing our application to misdetect that Npcap is already installed, and we had to introduce yet another clumsy workaround (namely to check if the npcap service is installed).
The expected behavior is that an installation should be transactional -- all or nothing. A failed installation should clean up any files or registry values (better yet, roll them back to a previous version if one was installed), leaving behind only installation logs.