Add public isStaleFileHandle() API, improve detection.

Add public isStaleFileHandle() API, improve detection.

Add a public API to the FileUtils to determine if an IOException is a
stale NFS file handle exception.  This will make it easier to detect
such errors, and interpret them consistently throughout the codebase.
This new API is a bit more lenient in its detection than the previous
detection, and should be able to detect some errors which previously
were not identified as stale file handle exceptions because they had the
word NFS in the error message.  Adjust the packfile handling code to use
this new API for detection.

Change-Id: I21f80014546ba1afec7335890e5ae79e7f521412
Signed-off-by: Martin Fick<mfick@codeaurora.org>