Groups: Explicitly display Screenname field filled out when displaying errors

[smallq-git]

Max username length for a group had been set to 35 symbols according to #123. However, max screenname length is 25 and if screenname field had not been filled during group creation, username will be used as screenname.

This leads to an error about incorrect length being returned, yet screenname field remains empty. This is confusing.

We need to explicitly show that username had been copied to fill empty screenname field when displaying length error.