Added new exception for when a transaction with supplied ID does not exists

Author: kai18

src/main/java/com/bigchaindb/exceptions/TransactionNotFoundException.java

@@ -0,0 +1,8 @@
+package com.bigchaindb.exceptions;
+
+public class TransactionNotFoundException extends Exception {
+
+  public TransactionNotFoundException(String message) {
+    super(message);
+  }
+}