memcacheMonitor: Remove try-catch block.

Since this is only used to show an exception with an invalid error
code, we may as well just handle the error in the default error
handler.

We also remove the unused $session variable.

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@2476 44740490-163a-0410-bde0-09ae8108e29a