Fix error logging for null-message exceptions

Fix error logging for null-message exceptions

Some exceptions (such as NullPointerException) can have null as their
getLocalizedMessage().  This meant that the render session could have
a result with a null message, but a non-null exception. This scenario
was not handled, for some error resulted in a failed result but no
details given.

Change-Id: I22c70cd09b3403e73e71ca971981b32026f00ad0