Fix bug where incorrect radar imagery files could be returned

If the `radar_id` in `get_available_radar()` is not numeric or not found in the database  it should produce an error.

R/get_radar_imagery.R

@@ -61,7 +61,7 @@ get_available_radar <- function(radar_id = "all") {
     )
   if (radar_id[1] == "all") {
     dat <- dat
-  } else if (as.numeric(radar_id) %in% dat$Radar_id) {
+  } else if (is.numeric(radar_id) && as.numeric(radar_id) %in% dat$Radar_id) {
     dat <- dat[dat$Radar_id %in% as.numeric(radar_id), ]
   } else{
     stop("radar_id not found")