Fixed hiding of setup exception due to a NPE in TransactionHandler

When the setup of a testcase fails due to some exception when setting up CDI context, the TransactionHandler throws a NullPointerException while trying to determine the transaction result (rollback or commit). This masks the original exception making it hard to figure out what goes wrong when building a context for test cases.

You can see the particular testcase here: https://github.com/dashorst/arquillian-transaction-npe-test 

The commit in this PR fixes the NPE and doesn't hide the original exception