Use the PurpleAir API to validate your Purple Air API Key.
Find more details on this function at https://api.purpleair.com/#api-keys-check-api-key.
Storing your key in the environment variable PURPLE_AIR_API_KEY is safer than storing it
in source code and is used by default in each PurpleAir function.
If the key is valid, a message is emitted and the input is invisibly returned;
invalid keys will throw an R error which utilizes information from the underlying http error
to inform the user.