Misleading error message when adding an issue to epic a second time

Summary

Attempting to add an issue to an epic a second time results in a misleading error message.

Steps to reproduce

  1. Create an epic.
  2. Add an issue to the epic.
  3. Using the 'Add an Issue' button, attempt to add the same issue again.

Example Project

Best tested locally using gdk.

What is the current bug behavior?

The message We can't find an issue that matches what you are looking for. is displayed.

Screencast_2019-10-02_12_37_32

What is the expected correct behavior?

Message displayed should be {Issue name} has already been added to {epic name}

Possible fixes

Test is at ee/spec/features/epics/epic_issues_spec.rb#L135

See #7355 (comment 224028682)