Invalid auEduPersonSharedToken (aepst) fails session creation

Currently aepst for Snapshot is limited to 27 char. 

This is correct to specification but ends up with the user receiving a nasty 500 if the value is longer than 27 char (i.e truly invalid).

We need an escape that checks for aepst being invalid in anyway and throws to a standalone page highlighting the fault with storing the normal snapshot data.

Explanation of aepst being broken and required format should be present on this page with a request to contact AAF support. 

Specification for aepst at http://wiki.aaf.edu.au/tech-info/attributes/auedupersonsharedtoken and http://www.aaf.edu.au/wp-content/uploads/2012/05/auEduPerson_attribute_vocabulary_v02-1-0.pdf