The following revision refers to this bug:
  https://chromium.googlesource.com/infra/infra/+/82b1a9a33f305eece6c628527779ef1e87620083

commit 82b1a9a33f305eece6c628527779ef1e87620083
Author: Nodir Turakulov <nodir@google.com>
Date: Sat Jan 06 01:15:11 2018

[buildbucket] get_task_def: handle BuilderNotFound

Respond with HTTP 404 on BuilderNotFound exception

R=iannucci@chromium.org

Bug:  798878 
Change-Id: I0fd70c8c465774636723f08cf4f588df96110ef8
Reviewed-on: https://chromium-review.googlesource.com/849496
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Nodir Turakulov <nodir@chromium.org>

[modify] https://crrev.com/82b1a9a33f305eece6c628527779ef1e87620083/appengine/cr-buildbucket/swarming/test/swarmbucket_api_test.py
[modify] https://crrev.com/82b1a9a33f305eece6c628527779ef1e87620083/appengine/cr-buildbucket/swarming/swarmbucket_api.py

https://apis-explorer.appspot.com/apis-explorer/?base=https://cr-buildbucket.appspot.com/_ah/api#p/swarmbucket/v1/swarmbucket.get_task_def?_h=1&resource=%257B%250A++%2522build_request%2522%253A+%250A++%257B%250A++++%2522bucket%2522%253A+%2522luci.chromium.try%2522%252C%250A++++%2522parameters_json%2522%253A+%2522%257B%255C%2522builder_name%255C%2522%253A+%255C%25221%255C%2522%257D%2522%250A++%257D%250A%257D&
returns 404