pdns_control: exit 1 on unknown command

Fixes #1295

(cherry picked from commit 72ab36f)

Author: rubenk

pdns/dynloader.cc

@@ -148,6 +148,10 @@ int main(int argc, char **argv)
     }
 
     string resp=D->receive();
+    if(resp.compare(0, 7, "Unknown") == 0) {
+      cerr<<resp<<endl;
+      return 1;
+    }
 
     cout<<resp<<endl;
   }