ndarray: improve error message

The ``.copy()`` step fails when attempting to copy an nd-array that does
not have a framework identifier set. That's okay, but the error message
should be more informative.

src/nb_ndarray.cpp

@@ -766,12 +766,19 @@ PyObject *ndarray_wrap(ndarray_handle *th, ndarray_framework framework,
     }
 
     if (copy) {
-        try {
-            o = o.attr("copy")();
-        } catch (std::exception &e) {
+        if (package.is_valid()) {
+            try {
+                o = o.attr("copy")();
+            } catch (std::exception &e) {
+                PyErr_Format(PyExc_RuntimeError,
+                             "nanobind::detail::ndarray_wrap(): copy failed: %s",
+                             e.what());
+                return nullptr;
+            }
+        } else {
             PyErr_Format(PyExc_RuntimeError,
-                         "nanobind::detail::ndarray_wrap(): copy failed: %s",
-                         e.what());
+                         "nanobind::detail::ndarray_wrap(): copy failed, must "
+                         "specify a target framework!");
             return nullptr;
         }
     }