Acknowledge missing images on units

[jduss4]

I was looking into why there was a discrepancy between the unit gallery:

and the unit viewer:

This is because the unit view is checking for images where file_exists is true, but in the case of poor n/a, it is only associated with images that have no actual file. We should note this when displaying information to the user. Two ideas:

• either filter by file_exists on gallery description display OR
• acknowledge the missing images in the unit tab pane