Fixing so that Cray C++ compiler is happy

The Cray C++ compiler was not recognizing the ``exit`` at the end of the the ``HDFS_ERROR`` and was generating warning at compile time. This commit fixes these problems by adding a return statement of the proper type at the end of ``HDFS_ERROR`` macro.

This has been tested on: gcc [4.7.2, 4.3.4], pgi, cray, and intel compilers.

@Kyle-B said he would review