clean up the layout & polished Mensa credit dialog

* don't crash if the nfc tag has been removed between triggering intent and connecting to the tag
* closes #21

app/src/main/java/com/codebutler/farebot/card/desfire/DesfireException.kt

@@ -4,6 +4,6 @@ package com.codebutler.farebot.card.desfire
  * Created by Jakob Wenzel on 16.11.13.
  */
 class DesfireException : Exception {
-    constructor(message: String) : super(message) {}
-    constructor(cause: Throwable) : super(cause) {}
+    constructor(message: String) : super(message)
+    constructor(cause: Throwable) : super(cause)
 }