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Templates splits made easy!
Panic less!

Try to avoid panicing on the middle of the program, in case that an
error occurs raise it to the top level in case that you can not do
anything to recover from it.
Fix Inde-out-of-range when running on the repo /

This commit fix a problem while splitting categories.

It seems that running polo without any argument on the repo root
directory will find the `README.md` file which doesn't belong to any
category, and that would break the rest of the rendering (the
`examples/` folder).