Missing file in `inputs` or `outputs` return an error instead of a warning

[RaphaelRibes]

Problem description

Why?

If you want to assure that your program did its thing correctly it's necessary, warnings can be ignored. I don't want this functionality to be mandatory but possible.

How ?

• Global config: Having a environment variable like MISSING_FILE_ERROR set to True.
• Task specific: like for no-default-feature = true in an environment, we can have missing_file_error = true