Attach modal shows a json error when no issues are available to attach

[DHaussermann]

When no issues are available to attach we should see s friendly message rather than a json error. This would be most common the plugin is locked to an org or repo and issue creation has been disabled there.

Steps:

• Connect a test user
• Save some other value in the Organization lock such as token-org (or find an org or repo that has issue creation disabled and point there)
• Make or locate a post
• Select the ... menu and Attach to GitHub

Observed: The modal shown an invalid json error