Sessions: delete sessions *after* they expire

not *before*. Derp
Missing sessions: give a proprer error

instead of the 'Cannot read property ast of undefined'.
Merge sessions--fix-stupid-expiration-bug (#5) into master

Sessions: delete sessions *after* they expire

Don't delete sessions *before* they expire. Derp!

Also, missing sessions: throw a proper error message
Instead of the 'Cannot read property ast of undefined'.