display informative error message when index is missing

[BereSoto]

Version

1.4.1

Reproduction link

https://codesandbox.io/s/vue

Steps to reproduce

• Create a folder without an index.md, index.comp.md inside or a [foldername].comp.md, [foldername.md] at the same level

What is expected ?

Just to work without index or to display an informative error that either of those options are needed (index or same name)

What is actually happening?

throws a "Atributes not found"

_{This bug report is available on Nuxt community (#c138)}