Related issues: Recover from error state

  1. When an error happens when adding a related issue, we should show the "Add" button as enabled:

Screen_Shot_2018-03-12_at_11.29.54

  1. The error message we are showing is not correct. That issue does exist.

  2. The loading spinner should be on the left of the text to match the rest of the loading buttons gitlab-ce~2024201