One error that occurs when packaging a multi-file addon into a ZIP is
zipping the contents of the addon, instead of the addon directory.
When installing the ZIP using addon_install, the files that should be
inside a directory instead get extracted into the top-level of the
script directory. There was also no user feedback about what went wrong.
Detect this case and fail with an error.
Ref: !117664
f87cc8ca8f