Poor UX when trying to create an issue if the repository does not permit it

[DHaussermann]

If a repo in bitbucket has issue creation disabled, the plugin UX does not handle this gracefully. We show an error message in the modal that should instead show a non-technical message to explain issue creation is not allowed keeping only the text from the message.

Steps:

• Login to your Bitbucket org such as https://bitbucket.org/DHaussermann
• Setup and configure the plugin oAuth using the instructions in the read-me
• Connect a test user
• Select any repo in the organization and disable issue creation
• Make or locate a post
• Select the ... menu and Create Bitbucket Issue
• Select the repository from the dropdown where issue creation has been disabled
• Click Submit

Observed: {"id":"","message":"failed to create issue: Sorry, either you don't have access to the repo DHaussermann/helloworld with the user dkh-sysadmin or it is no longer available","status_code":404}

[hanzei]

Same issue as https://github.com/mattermost/mattermost-plugin-github/issues/318. We can copy the solution from https://github.com/mattermost/mattermost-plugin-github/pull/333.