Fix pipeline subscription error flash message display

What does this MR do and why?

Closes #470442 (closed)

If a pipeline subscription failed to create, the errors returned can be an array of strings. The flash alert doesn't seem to render the strings in the array, so the flash alert contains blank text.

This change joins the array of strings to a single string. This approach is used elsewhere in the codebase.

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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

When bug is not fixed: Flash error shows in UI, but error text is blank.

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