make_apkg.py: Catch library and version errors

When creating (x86/ARM) packages, emit a warning if the
corresponding architecture libxwalkcore.so is not found.
If no packages have been created due to missing
libxwalkcore.so, abort with an error message.

Also, check if values <=0 are passed to --app-versionCode or
--app-versionCodeBase, and abort with an error message in case.