Improve error message in resources.Concat

[bep]

 <resources.Concat>: error calling Concat: slice []interface {} not supported in concat

The above is probably a slice of nil-- which means that the resources in the previous step wasn't found -- which would be a better error message.

[jmooring]

A slice consisting of two elements, a resource and a string, produces this error message:

error calling Concat: slice []interface {} not supported in concat

Reference: https://discourse.gohugo.io/t/resources-concat-failure/51354/7

[github-actions]

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