Code review comment for lp:~ubuntudotcom1/maas/event-api-test-409

Agree with Gavin's remark: I'm not sure having a specific template for 409 exceptions is a good idea: it mixes things up (but maybe the code could be fixed) but also it's mostly useless: most of the code that's susceptible to raise said exception has been "websocketified" already.

Additionally (and as discussed in Nuremberg), we should see if it isn't possible to put all this exception-handling logic in src/maasserver/middleware.py; this is where all the other exception-handling happens. This is mostly a problem with the previous branch (the one whose behavior this branches fixes), not this one.