fs: better error message when hole punch test fails
fs: better error message when hole punch test fails

See KUDU-984 for the actual documentation.

Change-Id: Iec75fa12a79c3159cd8e96118d23a6d483712b22
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/7464
Reviewed-by: Todd Lipcon <2c763d726a8c2f6afda9be7e0e56a2caa6bb5e6b@cloudera.com>
Tested-by: jenkins

InspurUSA/kudu,EvilMcJerkface/kudu,InspurUSA/kudu,helifu/kudu,helifu/kudu,andrwng/kudu,andrwng/kudu,helifu/kudu,InspurUSA/kudu,EvilMcJerkface/kudu,andrwng/kudu,cloudera/kudu,helifu/kudu,InspurUSA/kudu,andrwng/kudu,cloudera/kudu,andrwng/kudu,InspurUSA/kudu,InspurUSA/kudu,cloudera/kudu,helifu/kudu,cloudera/kudu,EvilMcJerkface/kudu,cloudera/kudu,helifu/kudu,andrwng/kudu,helifu/kudu,cloudera/kudu,EvilMcJerkface/kudu,helifu/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,andrwng/kudu,InspurUSA/kudu,InspurUSA/kudu,InspurUSA/kudu,cloudera/kudu,cloudera/kudu,EvilMcJerkface/kudu,andrwng/kudu,andrwng/kudu,cloudera/kudu,EvilMcJerkface/kudu,EvilMcJerkface/kudu,helifu/kudu