Pebble: ensure a better error message if someone tries to install a FW 1.x pbw

app/src/main/java/nodomain/freeyourgadget/gadgetbridge/devices/pebble/PBWReader.java

@@ -204,6 +204,9 @@ public class PBWReader {
                 }
                 app = new GBDeviceApp(appUUID, appName, appCreator, appVersion, appType);
             }
+            else if (!isFirmware) {
+                isValid = false;
+            }
         }
     }