Blank error text when adding private upstream pipeline subscription

Summary

Error flash message with blank text appears when trying to add a pipeline subscription where the upstream project is private.

Steps to reproduce

1. Create two new private projects: downstream and upstream
2. Ensure you have at least Developer permissions to the upstream project
3. In the Settings > CI/CD for the downstream project, attempt to add a Pipeline Subscription for upstream project

Example Project

What is the current bug behavior?

Flash error shows in UI, but error text is blank.

What is the expected correct behavior?

Flash error shows in UI, and should show the Ci::Subscriptions::Project model's validation error Upstream project needs to be public.

Relevant logs and/or screenshots

Output of checks

Results of GitLab environment info

Expand for output related to GitLab environment info

(For installations with omnibus-gitlab package run and paste the output of:
`sudo gitlab-rake gitlab:env:info`)

(For installations from source run and paste the output of:
`sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production`)

Results of GitLab application Check

Expand for output related to the GitLab application check

(For installations with omnibus-gitlab package run and paste the output of:
sudo gitlab-rake gitlab:check SANITIZE=true)
(For installations from source run and paste the output of:
sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true)
(we will only investigate if the tests are passing)

Possible fixes