Improve error handler for locations out of coverage

[christoph-morrison]

Buienradar provides data for netherlands and (most of) germany, but if data for a location outside the covered area is requested, it will still fail with a HTTP/404 error, but delivers also an error message:

// 20200413155827
// https://cdn-secure.buienalarm.nl/api/3.4/forecast.php?lat=46.480962&lon=11.305530&region=de&unit=mm/u

{
  "success": false,
  "reason": "not in coverage"
}

If a location is out of coverage, the module should handle this accordingly and should not just fail (mostly) silently.