LU-6058 test: log to stderr

In test-framework.sh's log() function print the message to stderr
instead of stdout. This ensures that the message will not be not
captured by command substitution. Similarly print bash stack traces to
stderr from error().

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: If9ca4af1c8d66f9bb3767f4fc3c04fb6966de915
Reviewed-on: http://review.whamcloud.com/13155
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>