box: fix func_index invalid return error message

Functional index extractor code used to raise an invalid
error message when the user-defined extractor function
returns a scalar instead of a table.

Closes #4553

(cherry picked from commit 260a3b3a)

src/box/key_list.c

@@ -144,7 +144,7 @@ key_list_iterator_next(struct key_list_iterator *it, const char **value)
 		diag_set(ClientError, ER_FUNC_INDEX_FORMAT, it->index_def->name,
 			 space ? space_name(space) : "",
 			 tt_sprintf("supplied key type is invalid: expected %s",
-				    field_type_strs[MP_ARRAY]));
+				    field_type_strs[FIELD_TYPE_ARRAY]));
 		return -1;
 	}
 	struct key_def *key_def = it->index_def->key_def;

test/engine/func_index.result

@@ -148,7 +148,7 @@ idx = s:create_index('idx', {func = box.func.invalidreturn0.id, parts = {{1, 'un
 s:insert({1})
  | ---
  | - error: 'Key format doesn''t match one defined in functional index ''idx'' of space
- |     ''withdata'': supplied key type is invalid: expected boolean'
+ |     ''withdata'': supplied key type is invalid: expected array'
  | ...
 idx:drop()
  | ---