typechecker: move Exception hierarchies to their own hhi file

Summary: ... including adding ErrorException

Reviewed By: @jwatzman

Differential Revision: D1714559

Signature: t1:1714559:1417631878:f0ed4e4590c534692476fed588dc40fde710c1b7
let travis cache packages retrieved via apt-get to speedup built

Summary: see http://docs.travis-ci.com/user/caching/#Caching-Ubuntu-packages
Closes https://github.com/facebook/hhvm/pull/4383

Reviewed By: @​svcscm

Differential Revision: D1719093

Pulled By: @fredemmott