explain3.js should assert that its writes succeed

The explain3.js test does not check that its writes succeed, but expects all documents to be inserted at the end of the test. If a stepdown occurs, causing writes to fail, then fewer documents will be present at the end of the test.

The test also does not assert anything about the behavior of an explain that is concurrent with a dropIndex. If we feel this is important to test, we should improve the test to make assertions, rather than just printing any exceptions the explain throws.