show stopper - don't add more errors after certain things happen

[Schrank]

define killing conditions which don’t need more explanation.

• coupon inactive
• wrong customer group
• net valid anymore
• not valid yet

@lfolco my customer wants to have a list of show stopper, which don't need more explanation. What do you think about them? I would add them to the repo, open a PR - if you like it we'll merge it, if not we don't.
If you don't find time to express your opinion (which is fine) then I'll decide ;-)

[lfolco]

This would be really nice actually, since sometimes we don't want to provide quite so much information necessarily.