bin/nerve: Update YAML exception handling to Psych semantics

As noted here https://github.com/airbnb/synapse/issues/52#issuecomment-45909582 updating the bin/nerve script to follow the new Psych exception semantics in Ruby 1.9.3+, matching what synapse is now doing.

Note that this now means nerve will only run in Ruby 1.9.3+. If you want a more generic solution that works across all Ruby versions, let me know, but I just followed the lead of what was done in synapse.