Do not attempt to build if there is no config file

[ollien]

As of now, if one attempts to run hugo in a directory without a config file, it will fail.

$ hugo
Error: Unable to locate config file or config directory. Perhaps you need to create a new site.
       Run `hugo help new` for details.

This addresses #5896. It is worth noting that if the directory has a go.mod file, building will still continue, as it is still considered a config file. I'm not 100% on how to deal with this, as it seems that we still do want these to be considered config files for Hugo's module system. It's a small edge case, but this will catch most cases.

[github-actions]

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