Add flash error for setAssignees failure

What does this MR do?

Add error state for setAssignees failure. The UX is currently being revised by @hollyreynolds

Repro:

• rails c
• Feature.enable(:swimlanes)
• Go to a group board http://127.0.0.1:3000/groups/h5bp/-/boards?group_by=epic
• Block the /graphql domain request to see the error
• Open the dropdown and select an assignee
• When its being set notice the error banner appears

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team