Improve error messages for unavailable quick actions

For most quick actions in https://docs.gitlab.com/ee/user/project/quick_actions.html, there will be scenarios where the command isn't available. Whether due to the state of the issue / MR / epic or due to permissions.

!27425 (closed) adds a generic error message when an action isn't available but we want to give a more detailed error to help the user.

@hollyreynolds I'll start with a few here to give you an idea of the possible reasons but I won't be able to do a complete list because there's too many 🙂

Quick Action	Availability condition
/tableflip	This is always available
/shrug	This is always available
/todo	Not available on new issues / MRs / epics. Not available when the user already has an existing Todo
/done	Only available when the user has an existing Todo
/subscribe	Not available on new issues / MRs / epics. Not available when the user is already subscribed
/unsubscribe	Only available when the user is already subscribed
/close	Available when issue / MR / epic is open and the user has update permissions
/reopen	Available when issue / MR / epic is closed and the user has update permissions
/title	Not available on new issues / MRs / epics. Available when user has update permissions