Add missing error handling to "Add list" dropdown in issue boards

Steps to reproduce

• navigate to issue boards (for example https://gitlab.com/gitlab-org/gitlab-ce/-/boards)
• switch browser to offline mode
• click Add list button

What is the current bug behavior?

The request to fetch dropdown content fails but the dropdown stays in loading state and no error is shown.

Reopening the dropdown shows no entries, no loading indicator, and no error.

boards-error-handling

What is the expected correct behavior?

An error is shown. When reopening the dropdown and no data is available, a new request is triggered.