Report stderr when untarring artifacts.

When tar fails it is incredibly hard to debug failures unless you have what it spewed to stderr.  Fortunately
it only spews to stderr when something goes wrong, so reporting stderr usually results in no messages.
The go library facilitates reading and writing tar files, but it doesn't assist with process of putting them onto the filesystem. There are enough corner cases involved with pushing things to the filesystem that I'd rather not deal with them.  (The FOWNER capability comes about because of some of this logic in tar.)

Using tar itself also guarantees us 100% compatibility with artifacts generated by tar.