Catching converters in case they throw.

ToHoudiniGeometryConverter throws in certain situtations, so we're catching those to avoid a seg fault.
Improving exception messages for failed PrimitiveVariables.
Adding extra catch to SceneCacheNode::scene just in case.

This function is used from every SceneCacheNode in Houdini to get the SceneInterface, so we want to be as careful as possible. Note that this still suffers from Memory Allocation Errors, which seems to be somehow controlled by Houdini directly using UT_NoMemHandler...
Improving UI for Expand/Collapse buttons.

I've set the enable flag for these buttons based on current expansion state, and added a more menacing comment to the Collapse description.
Merge pull request #269 from andrewkaufman/catchExceptions

Improved Exception handling in Houdini SceneCache nodes