Refine "Stop area {} belongs to multiple interchanges" error message

[alexey-zakharenkov]

Now, a "Stop area {} belongs to multiple interchanges" error message appears not only due to proclaimed reason, but also in the following cases:

1. A stop_area_group accidentally contains a stop_area several times, that is member list look like:
   stop_area_group: r1, r2, r3, r1
   where r1, r2, r3 are stop_area relations

2. Elements of stop_area are included also into stop_area_group. E.g.:
   stop_area 1 (r1): n111, w111, n112
   stop_area 2 (r2): n222, w222, n223
   stop_area_group: r1, r2, n111
   where n111 may be station/stop_position.

A relevant error message should be produced in these cases.