Exam mode crash

[Bunyod16]

A workstation can crash the greeter if someone tries to log in to exam mode when the network is down.

Steps to reproduce:

1. Put the workstation in exam mode.
2. Unplug the network cable.
3. Press the button to log in to the exam.

[FreekBes]

Instead of crashing the greeter will from the next release onwards display an error message on the bottom right of the screen, stating that the session could not be started.