Added try/catch in rviz plugin to prevent crash

When the rviz plugin received my RecognizedObjects message, it crashed rviz.

I traced the exception back as far as info_cache.cpp:62 where it initializes an object_recognition_core::db::ObjectDbParameters object.

The message I was sending was probably not what it was expecting, but that should still not crash rviz.  So I just added the try/catch you see here, and that makes it work fine for my purposes.