Allow writeErrors to be specified via failCommand

Drivers can currently use failCommand to have the server return {ok: 0} errors and errors in a writeConcernError document. Would it be possible to augment this functionality to return errors in a writeErrors array as well? The motivation for doing so is to test DRIVERS-555. One of the requirements for the project is to convert errors with code 50 (MaxTimeMSExpired) into a client-side TimeoutException. This error can be returned via {ok: 0}, writeErrors, or writeConcernError, so we'd like to be able to test all three formats via failpoints.

Acceptance Criteria: 

Add a new option to the failCommand command that allows users to inject an array of write errors.