config: give better error if a role is not a table

An attempt to use a non-table module as a role now reports a more
descriptive error:

> Unable to use module <...> as a role: expected table, got <...>

Fixes #10049

NO_DOC=The error is reported as before this commit as well as after it,
       but the error message is changed. The error message is not part
       of the API.