More robust initialization of ScannerContext

This commit is in conjunction with the "Fix missing status check in
lzo scanner." commit in the LZO repo. It provides a test case for the
LZO fix, and changes the ScannerContext initialization so it will fail
more gracefully instead of crashing.

Change-Id: Idcafeb3679a8fa54322d1ec31c6f1aba860e4e4f
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1680
Reviewed-by: Skye Wanderman-Milne <skye@cloudera.com>
Tested-by: jenkins