match error for import not in scope

When an import is ellipsized and not used in a scope in an exporting nonterminal, we get an error in terms of `match` instead of saying "import must be used in a scope"
```racket
(syntax-spec
  (nonterminal/exporting my-expr
    (foo body:racket-body ...)
    #:binding [(import body) ...]))
```
```
match: no matching clause for (imports ... (list (ellipsis ... (import ... #<pvar>))))
```