log more information when creating new FS layout
log more information when creating new FS layout

JD tried to run a TS with a data FS root that was actually a WAL FS root. It
had an instance metadata file, a wals subdirectory, but no data
subdirectory. He got a fairly opaque looking error about FS root creation.

Let's log a little bit more information when we couldn't open an existing FS
root. In his case, it would have said something about how the data/instance
metadata file could not be found.

Change-Id: I6adb4a7c04899fdbd2792c8cf9c3cdbc588fb1be
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/6025
Reviewed-by: Jean-Daniel Cryans <4bf4c125525b8623ac45dfd7774cbf531df19085@cloudera.com>
Tested-by: jenkins

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