140 dashboard improvements

[edarin]

Addressing these three issues:

• A server error (Invalid admin_id) is returned from the back-end when an admin unit without data is selected. It would be better to handle this more gracefully on the front-end or improve the error message from the back-end. The back-end error is actually caused by the renaming of administrative units as e.g. "Missing Donetska" and pcodes as e.g. "Missing UA14" here.
• The label for the age slider does not reflect the correct age for the selection. This is related to the expectation for an open-ended upper age class (e.g. 65+) when we currently have a closed upper age class (45-64). The issue is here.
• The legend needs an item to explain the yellow areas with missing data. See here.