handle missing decision tasks more gracefully

[bhearsum]

We currently get an unhandled exception when a decision task for a release cannot be found. On the frontend, this most likely shows up as a generic error, which is not very useful to users. We ought to turn it into a more actionable message (eg: "Couldn't create release; selected revision has no decision task").

The current full traceback is:

  File "flask/app.py", line 2525, in wsgi_app
    response = self.full_dispatch_request()
  File "flask/app.py", line 1822, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "flask_cors/extension.py", line 165, in wrapped_function
    return cors_after_request(app.make_response(f(*args, **kwargs)))
  File "flask/app.py", line 1820, in full_dispatch_request
    rv = self.dispatch_request()
  File "flask/app.py", line 1796, in dispatch_request
    return self.ensure_sync(self.view_functions[rule.endpoint])(**view_args)
  File "connexion/decorators/decorator.py", line 68, in wrapper
    response = function(request)
  File "connexion/decorators/uri_parsing.py", line 149, in wrapper
    response = function(request)
  File "connexion/decorators/validation.py", line 196, in wrapper
    response = function(request)
  File "connexion/decorators/response.py", line 112, in wrapper
    response = function(request)
  File "connexion/decorators/parameter.py", line 120, in wrapper
    return function(**kwargs)
  File "/usr/local/lib/python3.9/site-packages/shipit_api/admin/api.py", line 114, in add_release
    release.phases = generate_phases(release, common_input, verify_supported_flavors=True)
  File "/usr/local/lib/python3.9/site-packages/shipit_api/admin/tasks.py", line 203, in generate_phases
    decision_task_id = find_decision_task_id(release.repo_url, release.project, release.revision, release.product)
  File "/usr/local/lib/python3.9/site-packages/shipit_api/admin/tasks.py", line 59, in find_decision_task_id
    raise Exception(f"route {decision_task_route} exception {exc}")

[bhearsum]

same thing as #1337?

[bhearsum]

A few of us talked about this today. We all agreed that we should prioritize showing a useful error to the user in this situation.

(We also explicitly said that trying to handle this better, eg: by polling for the decision task until it is completed, is probably not necessary or worth prioritizing.)