WSOD when adding a term if ActiveMQ is down

[seth-shaw-unlv]

To reproduce:

1. Using a working Islandora 8 site...
2. Disable ActiveMQ
   1. vagrant ssh
   2. sudo systemctl stop activemq
3. Try to add a new taxonomy term:
   • WSOD: "The website encountered an unexpected error. Please try again later."
   • Watchdog reports two errors:
     • "Error publishing message: Could not connect to a broker (Host: localhost)"
     • "Error: Call to a member function getBeginFrame() on null in Stomp\States\ProducerTransactionState->initTransaction()..."
4. Re-enable activemq sudo systemctl restart activemq
5. Try to create a taxonomy term and everything works.

We should probably catch and report the issue rather than WSOD.

[seth-shaw-asu]

This issue was probably resolved with Islandora/islandora#868.