Improve support for traversal interrupts

Adding issue as requested in https://github.com/JanusGraph/janusgraph/pull/78#discussion_r99459762:

Traversal interrupts in at least the HBase backend currently lead to a JanusGraphException rather than a TraversalInterruptException during read in BackendTransaction. Goal here would be to refactor to support throwing and propagating a TraversalInterruptException directly.

Start by tracing traversal interrupt handling in BackendTransaction, especially the HBase backend path described in the issue. Confirm where interrupts become JanusGraphException and identify the existing TraversalInterruptException handling; done means the original exception is thrown and propagated directly during reads.

Written by the indexing model from the issue text.