Added more descriptive error messages

backend/src/main/kotlin/uk/co/neviyn/observationdatabase/controller/ObservationsController.kt

@@ -285,11 +285,12 @@ class ObservationsController {
         if (::mailer.isInitialized) {
             val data = getObservations(observationsRequest)
             if (data.isEmpty())
-                return false
+                throw ResponseStatusException(HttpStatus.BAD_REQUEST, "Request would send no data.")
             mailer.sendObservationData(data)
             return true
-        } else
+        } else {
             logger.error("Mailer has not been initialized.")
-        return false
+            throw ResponseStatusException(HttpStatus.SERVICE_UNAVAILABLE, "Mail is currently unavailable due to a server configuration error.")
+        }
     }
 }