Don't throw in mock autoloader

This behavior doesn't appear to be used at all.

This fixes HHVM compatibility: HHVM optimizes class_exists() to a
dedicated bytecode - as it's not a function call, it doesn't show up
in the backtrace. 100% of the tests pass with this change.
Merge pull request #2815 from fredemmott/fix-hhvm

Don't throw in mock autoloader