Handle special case where auEduPersonSharedToken is invalid

If the value is too long, we're going to hit problems trying to save
it in the database, so we need to just error out and show the problem
gracefully.

If the value is otherwise invalid, it will end up being changed anyway
which might cause collisions, so it's not a great idea to snapshot it.
Fix regular expression for validating auEduPersonSharedToken

The range A-z also includes ASCII chars between Z and a, which
includes:  [\]^_`
Added information to the error page about why auEduPersonSharedToken is invalid

Try to be as descriptive as possible about which requirements they have
broken, in the hope that this helps trap invalid configuration before
they deploy their production IdP.
Use the more Grailsy way of rendering a specific view + model

Just to be consistent, there's no functional benefit.
Merge pull request #2 from ausaccessfed/bugfix/#2-handle-invalid-aepst

Invalid auEduPersonSharedToken (aepst) fails session creation