TranslationHook was using a threading.local() to store an error
message however the threading.local() was being created in __init__
of the Hook. Hooks are not per request, so this wasn't really
working as planned.
Now, instead of using threading.local() at all, we just modify the
environ held by pecan (and later used by the
ParsableErrorMiddleware). environ is request-local.
This change feels a bit crufty because it is but because of the way
Exceptions are managed in the app there's not really any good way to do
it. We could consider changing the way Exceptions related to error
messages, but that would be a very large change.
Change-Id: I463059df28f291cea0644b1a9908a907f146ba1f
Closes-Bug: #1481244
9e07e7d971