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.

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>
(cherry picked from commit 427f53eea7f9c05e7b1913c91d57777d72bc30b2)

Change-Id: I04bebe60fbb275ca44caeed9b8696283c4ded36b
Reviewed-on: http://gerrit.openafs.org/7384
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>