Improve stability of db_stress

Summary:
Currently, whenever DB Verification fails we bail out by calling `exit(1)`. This is kind of bad since it causes unclean shutdown and spew of error log messages like:

    05:03:27 pthread lock: Invalid argument
    05:03:27 pthread lock: Invalid argument
    05:03:27 pthread lock: Invalid argument
    05:03:27 pthread lock: Invalid argument
    05:03:27 pthread lock: Invalid argument
    05:03:27 pthread lock: Invalid argument
    05:03:27 pthread lock: Invalid argument
    05:03:27 pthread lock: Invalid argument
    05:03:27 pthread lock: Invalid argument

This diff adds a new parameter that is set to true when verification fails. It can then use the parameter to bail out safely.

Test Plan: Casued artificail failure. Verified that exit was clean.

Reviewers: dhruba, haobo, ljin

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D18243