make_apkg.py: Catch library and version errors

When creating arm and x86 packages, emit a warning if the corresponding
libxwalkcore.so is not found. If neither one is found, and no package
can be created, 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.