Errors not properly reported out of render threads

[JosephCottam]

Some errors are swallowed silently, occasionally making it hard to tell if the execution failed or is just taking a long time.

[JosephCottam]

This may occur in transfer because all of the transfer work is done as a side-effect into a shared memory object. Need to check a return value or the thread-pool-state or similar for error codes.