airpol_get() does not throw an error with fake pollutants

```
test_that("airpol_get returns informative error messages", {
  # Fake pollutant should error
  expect_error(
    airpol_get(airpol = "FAKEPOLLUTANT", geo = "germany_1995", year = 1995),
    regexp = "not recognized"
  )
})
```