update: skip APKs that cannot be parsed instead of exiting
update: skip APKs that cannot be parsed instead of exiting

Some APKs can be corrupt or some system APKs do not have all the normal
info.  Instead of quitting, `fdroid update` skips the non-parsable APKs and
optionally deletes them if --delete-unknown is specified.

OneEducation/AppUniverse_Server,f-droid/fdroid-server,f-droid/fdroid-server,OneEducation/AppUniverse_Server,OneEducation/AppUniverse_Server,f-droid/fdroidserver,OneEducation/AppUniverse_Server,fdroidtravis/fdroidserver,f-droid/fdroid-server,f-droid/fdroidserver,fdroidtravis/fdroidserver,matlink/fdroidserver,OneEducation/AppUniverse_Server,fdroidtravis/fdroidserver,matlink/fdroidserver,f-droid/fdroid-server,f-droid/fdroidserver,matlink/fdroidserver,fdroidtravis/fdroidserver,f-droid/fdroidserver,matlink/fdroidserver,f-droid/fdroidserver,matlink/fdroidserver,f-droid/fdroid-server