Handle exceptions without message

app/src/main/java/eu/faircode/email/Helper.java

@@ -109,10 +109,10 @@ public class Helper {
 
     static String formatThrowable(Throwable ex) {
         StringBuilder sb = new StringBuilder();
-        sb.append(ex.getMessage());
+        sb.append(ex.getMessage() == null ? ex.getClass().getName() : ex.getMessage());
         Throwable cause = ex.getCause();
         while (cause != null) {
-            sb.append(" ").append(cause.getMessage());
+            sb.append(" ").append(cause.getMessage() == null ? cause.getClass().getName() : cause.getMessage());
             cause = cause.getCause();
         }
         return sb.toString();