fs: Report default storebehind when errors exist

fs: Report default storebehind when errors exist

After 904c9fbe, we no longer print out the default store asynchrony
when any of the supplied paths results in a pioctl error. However, if
just one (or a few) of the paths supplied results in an error (such
as, the path does not exist), this does not prevent us from reporting
the default value.

Instead, keep track of whether or not we have a valid value, and try
to determine the default if we haven't already by the end of
StoreBehindCmd, and print it out.

Change-Id: I16a5faed15141d0bb00ea9b6f991c0a8e404d4d5
Reviewed-on: http://gerrit.openafs.org/7376
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Derrick Brashear <shadow@dementix.org>