/series/import/request/{id}(seller.group): add validation for existing group

[0pdd]

The puzzle 857-aa59724c from #857 has to be resolved:

mystamps/src/main/java/ru/mystamps/web/controller/dto/ImportSellerForm.java

Line 40 in 95faa9a

// @todo #857 /series/import/request/{id}(seller.group): add validation for existing group

The puzzle was created by Slava Semushin on 15-Jul-18.