1051215: prevent an ArrayIndexOutOfBoundsException

While processing the exceptions in the ExceptionMapper, if an exception has no
stack trace, an ArrayIndexOutOfBoundsException was thrown losing the exception
being handled.
make test work on Java 6

Originally used a real NoStackException based on this ctor:

   Throwable(String message, Throwable cause,
             boolean enableSuppression, boolean writableStackTrace)

is a Java 7 only constructor. This caused the build to fail. Switching it to be
Java 6 compliant by using Mockito to mock a Throwable class and return null for
the getStackTrace() method.
Merge pull request #503 from candlepin/bz1051215

1051215: prevent an ArrayIndexOutOfBoundsException