Creating invitations fails due to invalid reviewer names

[Daniel Kulesz]

Environment:
- RevAger 1.3
- Ubuntu Linux 10.04
- OpenJDK Runtime Environment (IcedTea6 1.8) (6b18-1.8-0ubuntu1), OpenJDK 64-Bit Server VM (build 14.0-b16, mixed mode)

How to reproduce:
- Create a new review as moderator
- Add a person with a name that has i.e. a slash in it's name (e.g. "Student / Studentin")
- Try to export the invitation package

Expected behavior:
- RevAger warns you because of the the characters, or suggests a different name without those characters

Actual behavior:
- RevAger tries to create the invitation packages, and aborts the further execution once it gets to the package with the special character inside. A not-so-helpful, generic error message is shown.