Optionally make limit information visible to users

[jamezpolley]

_This tangentially relates to #141 and #136 _

We've had a metric buttton of queries coming in caused by the way limits and flags and vouchers make things appear and disappear at (as far as the consumer can tell) random.

Furthermore, it's really really really difficult to troubleshoot why an item is showing for everyone instead of just a small group, or why something isn't showing for anyone, or..

Both of these could be addressed if the "is met" mechanism could be enhanced to return messages as well as a boolean.

For instance: if Accommodation tickets have a Time/Stock limit that says that only 7 of this product can be sold, and the category as a whole is only available from 1 Nov -> 20 Dec, it would be very handy to display that to the end user - both in advance (so that they can see that 6/7 have been sold and they need to hurry up) and after the fact (so that they can see that this product used to be available but is now sold out).