A better error dialog is needed when Apple's installer fails with return code 1

[MagerValp]

Description of issue

When Apple's installer fails AutoDMG reports it with a generic Installation of 'InstallInfo.plist' failed with return code 1. See the log for details. A dialog that explains the error better is needed, as it's Apple's installer failing, not AutoDMG.

Observed causes of this behavior so far:

• Having the OS installer app on the system volume (e.g. the default location /Applications), see FAQ #0.
• Incomplete OS installer, see FAQ #9.
• Flaky dmg mounter. Sometimes macOS just fails to mount dmgs until you reboot.
• Some systems seem to trigger an unidentified bug in the installer, where it fails to accept the target volume as valid for no apparent reason.

If you want help, please join the macadmins slack. If you want the bugs fixed, please file feedback with Apple.