hammer: common/obj_bencher.cc: make verify error fatal
common/obj_bencher.cc: make verify error fatal
When run without "--no-verify", all verification errors are noted,
but they are not forwarded/reported anywhere else but to cerr, which
will cause automated testing to ignore them. Make seq_read_bench and
rand_read_bench return -EIO on any verification error which will,
in turn, return it back to caller.