Fix uncaught exception in LuaStandalone

If the interpreter exits before the end of the page, then the call to
Scribunto_LuaStandaloneEngine::getLimitReport() throws an uncaught
exception when it tries to access the interpreter. Catch it.

Change-Id: I7ce4f09b1b2206f13ab0f422de35e0b69a3b24d5