Check the chosen command for existence

config/config.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"log"
 	"os"
+	"os/exec"
 	"strings"
 
 	"gopkg.in/yaml.v2"
@@ -147,6 +148,12 @@ func (c *Config) UpdateFromJSON(j []byte) error {
 			}
 		}
 
+		// check the command exists
+		_, err := exec.LookPath(newConfig.DownloadProfiles[i].Command)
+		if err != nil {
+			return fmt.Errorf("Could not find %s on the path", newConfig.DownloadProfiles[i].Command)
+		}
+
 	}
 
 	*c = newConfig