Improved error message for bad area name

[ajparsons]

Given you know through show_area_values all the valid values, can you throw an error message if an input to subset_coords is outside that?

For instance, if instead of saying 'Croydon' I say 'croydon', the error I get is a 502 error when it actually tries to query the API.

It'd be more helpful if subset_coords told me in advance something like:

'croydon' is not a valid setset, please run 'show_area_values()' for a list of valid areas.

Where this might especially trip people up is if you put multiple areas in, spelt one of them wrong, you would still get data, but might not immediately spot you had less than you wanted.