ceph_objectstore_tool: Add assert on errors so failures get back to teut...

...hology

Signed-off-by: David Zafman <dzafman@redhat.com>
As-is, the error message this generates won't be terribly useful. Many tasks would do something like:
```
raise RuntimeError("Test failed because blah")
```

It looks like this task tends to use this pattern:
```
assert did_not_fail, "Test failed because blah"
```

I'm also not sure it's in the right place. I think it should probably go a few lines up in the 'else' clause after the logging call.